Class B amplifier-Working

Unlike the Class A amplifier mode of operation above that uses a single transistor for its output power stage, the Class B Amplifier uses two complimentary transistors (an NPN and a PNP) for each half of the
output waveform. One transistor conducts for one-half of the signal waveform while the other conducts for the other or opposite half of the signal waveform. This means that each transistor spends half of its time in the active region and half its time in the cut-off region thereby amplifying only 50% of the input signal.

Class B operation has no direct DC bias voltage like the class A amplifier, but instead the transistor only conducts when the input signal is greater than the base-emitter voltage and for silicon devices is about 0.7v. Therefore, at zero input there is zero output. This then results in only half the input signal being presented at the amplifiers output giving a greater amount of amplifier efficiency as shown below.

Class B- Circuit Diagram

In a class B amplifier, no DC current is used to bias the transistors, so for the output transistors to start to conduct each half of the waveform, both positive and negative, they need the base-emitter voltage Vbe to be greater than the 0.7v required for a bipolar transistor to start conducting. Then the lower part of the output waveform which is below this 0.7v window will not be reproduced accurately resulting in a distorted area of the output waveform as one transistor turns "OFF" waiting for the other to turn back "ON". The result is that there is a small part of the output waveform at the zero voltage cross over point which will be distorted. This type of distortion is called Crossover Distortion

Class AB amplifier-Worrking

The Class AB Amplifier is a compromise between the Class A and the Class B configurations above. While Class AB operation still uses two complementary transistors in its output stage a very small biasing voltage is applied to the Base of the transistor to bias it close to the Cut-off region when no input signal is
present.

An input signal will cause the transistor to operate as normal in its Active region thereby eliminating any crossover distortion which is present in class B configurations. A small Collector current will flow when there is no input signal but it is much less than that for the Class A amplifier configuration. This means then that the transistor will be "ON" for more than half a cycle of the waveform. This type of amplifier configuration improves both the efficiency and linearity of the amplifier circuit compared to a pure Class A configuration.

Class AB- Circuit Diagram

Class AB Output Waveform

The class of operation for an amplifier is very important and is based on the amount of transistor bias required for operation as well as the amplitude required for the input signal. Amplifier classification takes into account the portion of the input signal in which the transistor conducts as well as determining both the efficiency and the amount of power that the switching transistor both consumes and dissipates in the form of wasted heat.

Class C amplifier - Working

In class C operation, collector current flows for less than one half cycle of the input signal.

The class C operation is achieved by reverse biasing the emitter-base junction,
which sets the dc operating point below cutoff and allows only the portion of the input signal that

overcomes the reverse bias to cause collector current flow. if an input signal amplitude is increased to the point that the transistor goes into saturation and cutoff, it is then called an OVERDRIVEN amplifier. 
Working:

During the positive period of the input signal (On stage)During the positiv period of the input signal the transistor will conduct (On-state). You can imagin that the transistor is a switch which connects the emitter with the collector. 
What will happend now is that the current I1 (red) flow through the coil and then into the transistor and down to ground. A magnetic field builds up in the coil depending on the magnitude of the current. At the same time the voltage over the capacitor discharge through the resistor making another current flow I2 (blue) also through the transistor. The I2 current passes the resistor (antenna) which radiate the energy.

During the negative period of the input signal (Off stage)
During the negativ period of the input signal the transistor will not conduct (Off-state). You can imagin that the transistor is an open switch. No current can pass through the collector to the emitter. 
The magnetic filed which was build upp in the coil will now collaps and generate a current I1 (red) which will flow through the capacitor and into the resistor (antenna).

Non-inverting Amplifier

The Non-inverting Amplifier

The second basic configuration of an operational amplifier circuit is that of a Non-inverting Amplifier. In this configuration, the input voltage signal, ( Vin ) is applied directly to the non-inverting ( + ) input terminal
which means that the output gain of the amplifier becomes "Positive" in value in contrast to the "Inverting Amplifier" circuit we saw in the last tutorial whose output gain is negative in value. The result of this is that the output signal is "in-phase" with the input signal.

Feedback control of the non-inverting amplifier is achieved by applying a small part of the output voltage signal back to the inverting ( - ) input terminal via a Rƒ - R2 voltage divider network, again producing negative feedback. This closed-loop configuration produces a non-inverting amplifier circuit with very good stability, a very high input impedance, Rin approaching infinity, as no current flows into the positive input terminal, (ideal conditions) and a low output impedance, Rout as shown below.

Non-inverting Amplifier Configuration

In the previous Inverting Amplifier tutorial, we said that "no current flows into the input" of the amplifier and that "V1 equals V2". This was because the junction of the input and feedback signal ( V1 ) are at the same potential. In other words the junction is a "virtual earth" summing point. Because of this virtual earth node the resistors, Rƒ and R2 form a simple potential divider network across the non-inverting amplifier with the voltage gain of the circuit being determined by the ratios of R2 and Rƒ as shown below.

Equivalent Potential Divider Network

Then using the formula to calculate the output voltage of a potential divider network, we can calculate the closed-loop voltage gain ( A _V ) of the Non-inverting Amplifier as follows:

Then the closed loop voltage gain of a Non-inverting Amplifier is given as:

We can see from the equation above, that the overall closed-loop gain of a non-inverting amplifier will always be greater but never less than one (unity), it is positive in nature and is determined by the ratio of the values of Rƒ and R2. If the value of the feedback resistor Rƒ is zero, the gain of the amplifier will be exactly equal to one (unity). If resistor R2 is zero the gain will approach infinity, but in practice it will be limited to the operational amplifiers open-loop differential gain, ( Ao ).

We can easily convert an inverting operational amplifier configuration into a non-inverting amplifier configuration by simply changing the input connections as shown.

Voltage Follower (Unity Gain Buffer)

If we made the feedback resistor, Rƒ equal to zero, (Rƒ = 0), and resistor R2 equal to infinity, (R2 = ∞), then the circuit would have a fixed gain of "1" as all the output voltage would be present on the inverting input terminal (negative feedback). This would then produce a special type of the non-inverting amplifier circuit called a Voltage Follower or also called a "unity gain buffer".

As the input signal is connected directly to the non-inverting input of the amplifier the output signal is not inverted resulting in the output voltage being equal to the input voltage, Vout = Vin. This then makes thevoltage follower circuit ideal as a Unity Gain Buffer circuit because of its isolation properties as impedance or circuit isolation is more important than amplification while maintaining the signal voltage. The input impedance of the voltage follower circuit is very high, typically above 1MΩ as it is equal to that of the operational amplifiers input resistance times its gain ( Rin x Ao ). Also its output impedance is very low since an ideal op-amp condition is assumed.

Voltage Follower

In this non-inverting circuit configuration, the input impedance Rin has increased to infinity and the feedback impedance Rƒ reduced to zero. The output is connected directly back to the negative inverting input so the feedback is 100% and Vin is exactly equal to Vout giving it a fixed gain of 1 or unity. As the input voltage Vin is applied to the non-inverting input the gain of the amplifier is given as:

Since no current flows into the non-inverting input terminal the input impedance is infinite (ideal op-amp) and also no current flows through the feedback loop so any value of resistance may be placed in the feedback loop without affecting the characteristics of the circuit as no voltage is dissipated across it, zero current flows, zero voltage drop, zero power loss.

Since the input current is zero giving zero input power, the voltage follower can provide a large power gain. However in most real unity gain buffer circuits a low value (typically 1kΩ) resistor is required to reduce any offset input leakage currents, and also if the operational amplifier is of a current feedback type.

The voltage follower or unity gain buffer is a special and very useful type of Non-inverting amplifiercircuit that is commonly used in electronics to isolated circuits from each other especially in High-order state variable or Sallen-Key type active filters to separate one filter stage from the other. Typical digital buffer IC's available are the 74LS125 Quad 3-state buffer or the more common 74LS244 Octal buffer.

One final thought, the output voltage gain of the voltage follower circuit with closed loop gain is Unity, the voltage gain of an ideal operational amplifier with open loop gain (no feedback) is Infinite. Then by carefully selecting the feedback components we can control the amount of gain produced by an operational amplifier anywhere from one to infinity.

Thus far we have analysed an inverting and non-inverting amplifier circuit that has just one input signal,Vin. In the next tutorial about Operational Amplifiers, we will examine the effect of the output voltage,Vout by connecting more inputs to the amplifier. This then produces another common type of operational amplifier circuit called a Summing Amplifier which can be used to "add" together the voltages present on its inputs.

Inverting Amplifier

The Inverting Amplifier

We saw in the last tutorial that the Open Loop Gain, ( Avo ) of an ideal operational amplifier can be very high, as much as 1,000,000 (120dB) or more. However, this very high gain is of no real use to us as it
makes the amplifier both unstable and hard to control as the smallest of input signals, just a few micro-volts, (μV) would be enough to cause the output voltage to saturate and swing towards one or the other of the voltage supply rails losing complete control of the output.

As the open loop DC gain of an operational amplifier is extremely high we can therefore afford to lose some of this high gain by connecting a suitable resistor across the amplifier from the output terminal back to the inverting input terminal to both reduce and control the overall gain of the amplifier. This then produces and effect known commonly as Negative Feedback, and thus produces a very stable Operational Amplifier based system.

Negative Feedback is the process of "feeding back" a fraction of the output signal back to the input, but to make the feedback negative, we must feed it back to the negative or "inverting input" terminal of the op-amp using an external Feedback Resistor called Rƒ. This feedback connection between the output and the inverting input terminal forces the differential input voltage towards zero.

This effect produces a closed loop circuit to the amplifier resulting in the gain of the amplifier now being called its Closed-loop Gain. Then a closed-loop inverting amplifier uses negative feedback to accurately control the overall gain of the amplifier, but at a cost in the reduction of the amplifiers bandwidth.

This negative feedback results in the inverting input terminal having a different signal on it than the actual input voltage as it will be the sum of the input voltage plus the negative feedback voltage giving it the label or term of a Summing Point. We must therefore separate the real input signal from the inverting input by using an Input Resistor, Rin.

As we are not using the positive non-inverting input this is connected to a common ground or zero voltage terminal as shown below, but the effect of this closed loop feedback circuit results in the voltage potential at the inverting input being equal to that at the non-inverting input producing a Virtual Earthsumming point because it will be at the same potential as the grounded reference input. In other words, the op-amp becomes a "differential amplifier".

Inverting Amplifier Configuration

In this Inverting Amplifier circuit the operational amplifier is connected with feedback to produce a closed loop operation. For ideal op-amps there are two very important rules to remember about inverting amplifiers, these are: "no current flows into the input terminal" and that "V1 equals V2", (in real world op-amps both of these rules are broken).

This is because the junction of the input and feedback signal ( X ) is at the same potential as the positive ( + ) input which is at zero volts or ground then, the junction is a "Virtual Earth". Because of this virtual earth node the input resistance of the amplifier is equal to the value of the input resistor, Rin and the closed loop gain of the inverting amplifier can be set by the ratio of the two external resistors.

We said above that there are two very important rules to remember about Inverting Amplifiers or any operational amplifier for that matter and these are.

• 1.  No Current Flows into the Input Terminals
•  
• 2.  The Differential Input Voltage is Zero as V1 = V2 = 0 (Virtual Earth)

Then by using these two rules we can derive the equation for calculating the closed-loop gain of an inverting amplifier, using first principles.

Current ( i ) flows through the resistor network as shown.

Then, the Closed-Loop Voltage Gain of an Inverting Amplifier is given as.

and this can be transposed to give Vout as:

Linear Output

The negative sign in the equation indicates an inversion of the output signal with respect to the input as it is 180^o out of phase. This is due to the feedback being negative in value.

The equation for the output voltage Vout also shows that the circuit is linear in nature for a fixed amplifier gain as Vout = Vin x Gain. This property can be very useful for converting a smaller sensor signal to a much larger voltage.

Another useful application of an inverting amplifier is that of a "transresistance amplifier" circuit. ATransresistance Amplifier also known as a "transimpedance amplifier", is basically a current-to-voltage converter (Current "in" and Voltage "out"). They can be used in low-power applications to convert a very small current generated by a photo-diode or photo-detecting device etc, into a usable output voltage which is proportional to the input current as shown.

Transresistance Amplifier Circuit

The simple light-activated circuit above, converts a current generated by the photo-diode into a voltage. The feedback resistor Rƒ sets the operating voltage point at the inverting input and controls the amount of output. The output voltage is given as Vout = I_s x Rƒ. Therefore, the output voltage is proportional to the amount of input current generated by the photo-diode.

Example No1

Find the closed loop gain of the following inverting amplifier circuit.

Using the previously found formula for the gain of the circuit

we can now substitute the values of the resistors in the circuit as follows,

Rin = 10kΩ  and  Rƒ = 100kΩ.

and the gain of the circuit is calculated as    -Rƒ/Rin = 100k/10k = 10.

therefore, the closed loop gain of the inverting amplifier circuit above is given 10 or 20dB (20log(10)).

Example No2

The gain of the original circuit is to be increased to 40 (32dB), find the new values of the resistors required.

Assume that the input resistor is to remain at the same value of 10KΩ, then by re-arranging the closed loop voltage gain formula we can find the new value required for the feedback resistor Rƒ.

   Gain = -Rƒ/Rin

therefore,   Rƒ = Gain x Rin

  Rƒ = 40 x 10,000

  Rƒ = 400,000 or 400KΩ

The new values of resistors required for the circuit to have a gain of 40 would be,

 Rin = 10KΩ  and  Rƒ = 400KΩ.

The formula could also be rearranged to give a new value of Rin, keeping the same value of Rƒ.

One final point to note about the Inverting Amplifier configuration for an operational amplifier, if the two resistors are of equal value, Rin = Rƒ  then the gain of the amplifier will be -1 producing a complementary form of the input voltage at its output as Vout = -Vin. This type of inverting amplifier configuration is generally called a Unity Gain Inverter of simply an Inverting Buffer.

Operational Amplifiers

Ideal Operational Amplifiers

As well as resistors and capacitors, Operational Amplifiers, or Op-amps as they are more commonly called, are one of the basic building blocks of Analogue Electronic Circuits. Operational amplifiers are
linear devices that have all the properties required for nearly ideal DC amplification and are therefore used extensively in signal conditioning, filtering or to perform mathematical operations such as add, subtract, integration and differentiation.

An ideal Operational Amplifier is basically a three-terminal device which consists of two high impedance inputs, one called the Inverting Input, marked with a negative or "minus" sign, ( - ) and the other one called the Non-inverting Input, marked with a positive or "plus" sign ( + ).

The third terminal represents the op-amps output port which can both sink and source either a voltage or a current. In a linear operational amplifier, the output signal is the amplification factor, known as the amplifiers gain ( A ) multiplied by the value of the input signal and depending on the nature of these input and output signals, there can be four different classifications of operational amplifier gain.

• Voltage  – Voltage "in" and Voltage "out"
•  
• Current  – Current "in" and Current "out"
•  
• Transconductance  – Voltage "in" and Current "out"
•  
• Transresistance  – Current "in" and Voltage "out"

Since most of the circuits dealing with operational amplifiers are voltage amplifiers, we will limit the tutorials in this section to voltage amplifiers only, (Vin and Vout).

The amplified output signal of an Operational Amplifier is the difference between the two signals being applied to the two inputs. In other words the output signal is a differential signal between the two inputs and the input stage of an Operational Amplifier is in fact a differential amplifier as shown below.

Differential Amplifier

The circuit below shows a generalized form of a differential amplifier with two inputs marked V1 and V2. The two identical transistors TR1 and TR2 are both biased at the same operating point with their emitters connected together and returned to the common rail, -Vee by way of resistor Re.

Differential Amplifier

The circuit operates from a dual supply+Vcc and -Vee which ensures a constant supply. The voltage that appears at the output, Vout of the amplifier is the difference between the two input signals as the two base inputs are in anti-phase with each other. So as the forward bias of transistor,TR1 is increased, the forward bias of transistor TR2 is reduced and vice versa. Then if the two transistors are perfectly matched, the current flowing through the common emitter resistor, Re will remain constant.

Like the input signal, the output signal is also balanced and since the collector voltages either swing in opposite directions (anti-phase) or in the same direction (in-phase) the output voltage signal, taken from between the two collectors is, assuming a perfectly balanced circuit the zero difference between the two collector voltages. This is known as the Common Mode of Operation with the common mode gain of the amplifier being the output gain when the input is zero.

Ideal Operational Amplifiers also have one output (although there are ones with an additional differential output) of low impedance that is referenced to a common ground terminal and it should ignore any common mode signals that is, if an identical signal is applied to both the inverting and non-inverting inputs there should no change to the output. However, in real amplifiers there is always some variation and the ratio of the change to the output voltage with regards to the change in the common mode input voltage is called the Common Mode Rejection Ratio or CMRR.

Operational Amplifiers on their own have a very high open loop DC gain and by applying some form ofNegative Feedback we can produce an operational amplifier circuit that has a very precise gain characteristic that is dependant only on the feedback used. An operational amplifier only responds to the difference between the voltages on its two input terminals, known commonly as the "Differential Input Voltage" and not to their common potential. Then if the same voltage potential is applied to both terminals the resultant output will be zero. An Operational Amplifiers gain is commonly known as theOpen Loop Differential Gain, and is given the symbol (A_o).

Equivalent Circuit for Ideal Operational Amplifiers

Op-amp Idealized Characteristics

PARAMETER	IDEALIZED CHARACTERISTIC
Open Loop Gain, (Avo)	Infinite - The main function of an operational amplifier is to amplify the input signal and the more open loop gain it has the better. Open-loop gain is the gain of the op-amp without positive or negative feedback and for an ideal amplifier the gain will be infinite but typical real values range from about 20,000 to 200,000.
Input impedance, (Zin)	Infinite - Input impedance is the ratio of input voltage to input current and is assumed to be infinite to prevent any current flowing from the source supply into the amplifiers input circuitry (Iin =0). Real op-amps have input leakage currents from a few pico-amps to a few milli-amps.
Output impedance, (Zout)	Zero - The output impedance of the ideal operational amplifier is assumed to be zero acting as a perfect internal voltage source with no internal resistance so that it can supply as much current as necessary to the load. This internal resistance is effectively in series with the load thereby reducing the output voltage available to the load. Real op-amps have output-impedance in the 100-20Ω range.
Bandwidth, (BW)	Infinite - An ideal operational amplifier has an infinite frequency response and can amplify any frequency signal from DC to the highest AC frequencies so it is therefore assumed to have an infinite bandwidth. With real op-amps, the bandwidth is limited by the Gain-Bandwidth product (GB), which is equal to the frequency where the amplifiers gain becomes unity.
Offset Voltage, (Vio)	Zero - The amplifiers output will be zero when the voltage difference between the inverting and the non-inverting inputs is zero, the same or when both inputs are grounded. Real op-amps have some amount of output offset voltage.

From these "idealized" characteristics above, we can see that the input resistance is infinite, so no current flows into either input terminal (the "current rule") and that the differential input offset voltage is zero (the "voltage rule"). It is important to remember these two properties as they will help us understand the workings of the Operational Amplifier with regards to the analysis and design of op-amp circuits.

However, real Operational Amplifiers such as the commonly available uA741, for example do not have infinite gain or bandwidth but have a typical "Open Loop Gain" which is defined as the amplifiers output amplification without any external feedback signals connected to it and for a typical operational amplifier is about 100dB at DC (zero Hz). This output gain decreases linearly with frequency down to "Unity Gain" or 1, at about 1MHz and this is shown in the following open loop gain response curve.

Open-loop Frequency Response Curve

From this frequency response curve we can see that the product of the gain against frequency is constant at any point along the curve. Also that the unity gain (0dB) frequency also determines the gain of the amplifier at any point along the curve. This constant is generally known as the Gain Bandwidth Product or GBP.

Therefore, GBP = Gain x Bandwidth or A x BW.

For example, from the graph above the gain of the amplifier at 100kHz = 20dB or 10, then the

GBP = 100,000Hz x 10 = 1,000,000.

Similarly, a gain at 1kHz = 60dB or 1000, therefore the

GBP = 1,000 x 1,000 = 1,000,000. The same!.

The Voltage Gain (A) of the amplifier can be found using the following formula:

and in Decibels or (dB) is given as:

An Operational Amplifiers Bandwidth

The operational amplifiers bandwidth is the frequency range over which the voltage gain of the amplifier is above 70.7% or -3dB (where 0dB is the maximum) of its maximum output value as shown below.

Here we have used the 40dB line as an example. The -3dB or 70.7% of Vmax down point from the frequency response curve is given as 37dB. Taking a line across until it intersects with the main GBP curve gives us a frequency point just above the 10kHz line at about 12 to 15kHz. We can now calculate this more accurately as we already know the GBP of the amplifier, in this particular case 1MHz.

Example No1.

Using the formula 20 log (A), we can calculate the bandwidth of the amplifier as:

37 = 20 log A   therefore, A = anti-log (37 ÷ 20) = 70.8

GBP ÷ A = Bandwidth,  therefore, 1,000,000 ÷ 70.8 = 14,124Hz, or 14kHz

Then the bandwidth of the amplifier at a gain of 40dB is given as 14kHz as previously predicted from the graph.

Example No2.

If the gain of the operational amplifier was reduced by half to say 20dB in the above frequency response curve, the -3dB point would now be at 17dB. This would then give the operational amplifier an overall gain of 7.08, therefore A = 7.08.

If we use the same formula as above, this new gain would give us a bandwidth of approximately141.2kHz, ten times more than at the 40dB point. It can therefore be seen that by reducing the overall "open loop gain" of an operational amplifier its bandwidth is increased and visa versa. In other words, an operational amplifiers bandwidth is proportional to its gain. Also, this -3dB point is generally known as the "half power point", as the output power of the amplifier is at half its maximum value at this value.

Operational Amplifiers Summary

We know now that an Operational amplifiers is a very high gain DC differential amplifier that uses one or more external feedback networks to control its response and characteristics. We can connect external resistors or capacitors to the op-amp in a number of different ways to form basic "building Block" circuits such as, Inverting, Non-Inverting, Voltage Follower, Summing, Differential, Integrator and Differentiator type amplifiers.

Op-amp Symbol

An "ideal" or perfect Operational Amplifier is a device with certain special characteristics such as infinite open-loop gain Ao, infinite input resistance Rin, zero output resistance Rout, infinite bandwidth 0 to ∞and zero offset (the output is exactly zero when the input is zero).

There are a very large number of operational amplifier IC's available to suit every possible application from standard bipolar, precision, high-speed, low-noise, high-voltage, etc in either standard configuration or with internal JFET transistors. Operational amplifiers are available in IC packages of either single, dual or quad op-amps within one single device. The most commonly available and used of all operational amplifiers in basic electronic kits and projects is the industry standard μA-741.

TRANSISTORS

• TRANSISTORS

• Bipolar Junction Transistor Model

• The Ideal Bipolar Junction Transistor

The Ideal Bipolar Junction Transistor

Because the current gain is typically unknown or varies greatly with temperature, time, collector–emitter

potential, and other factors, good designs should not depend on it. In this laboratory, we assume that is

sufficiently large (i.e.,amplification ≫ 1, where amplification ≈ 100 in our laboratory) so that

iB ≈ 0 and iC ≈ iE.

These simple rules are similar to the rules we use with operational amplifiers. The analysis approach usually

follows these steps:

1. Calculate the transistor base potential vB by assuming that no current enters the base (i.e., iB ≈ 0).

2. Calculate the potential vE at the emitter of the transistor using vB. For an npn transistor,

vE = vB − 0.65V,

and for a pnp transistor,

vE = vB + 0.65V.

3. Calculate the emitter current iE using the emitter voltage vE and the rest of the circuit.

4. Assume that iC ≈ iE and analyze the rest of the circuit.

• Because we know vE, we usually know iE as well. So our iE dictates what iC should be.

However, keep these notes in mind.

• For an npn transistor, active mode requires vC − vE > 0.2V. For a pnp transistor, active mode

requires vE − vC > 0.2V. If this condition is violated, the transistor is saturated, and the analysis

cannot continue using these simple rules. In design problems, change parameters (e.g., resistors, supply

rails, etc.) to prevent saturation.

• Sometimes it’s easier to find vE first and use it to calculate vB.

• How “small” iB must be to neglect its effect depends on the circuit. In particular, iB × RB must be

very small, where RB is the the Th´evenin equivalent resistance looking out of the transistor base.

Bipolar Junction Transistor Model

A bipolar junction transistor (BJT) can be in three modes:

Transistor acts like an open switch between collector and emitter (i.e.,
collector–emitter “resistance” is infinite).


Transistor acts like a dynamic resistor between collector and emitter that
adjusts its resistance in order to keep collector current at a set level (i.e.,

collector–emitter resistance is finite and positive).

Transistor acts like a closed switch between collector and emitter (i.e.,
collector–emitter “resistance” is very low).


In the active mode, the transistor adjusts the collector current to be a version of the base current amplified
by some constant > 0. If the base current falls to 0, the transistor enters cutoff mode and shuts off. When
the base current rises too far, the transistor loses its ability to decrease the collector–emitter resistance
to linearly increase the collector current. In this case, the transistor enters saturation mode. To keep the
transistor out of saturation mode, the collector and emitter should be separated by at least 0.2V.
A simple model for the operation of a transistor in active mode is shown in Figure. It requires knowing
the current gain in order to design the circuit. In both of these models,
iC = iB and iE = ( + 1)iB,
and the emitter is separated from the base by a diode. In order for this diode to conduct current, it must
be forward biased with 0.65V1.

TRANSISTORS

Transistors can be regarded as a type of switch, as can many electronic components. They are used in a variety of circuits and you will find that it is rare that a circuit built in a school Technology Department does not contain at least one transistor. They are central to electronics and there are two main types; NPN and PNP. Most circuits tend to use NPN. There are hundreds of transistors which work at different voltages but all of them fall into these two categories.

Diode

• Introduction
• Diode equation
• General Diode Specifications
• Types of diodes
• Rectifier diodes
• Photodiode
• LED

LED

A Light-Emitting Diode (LED) in essence is a P-N junction solid-state semiconductor
diode that emits light when a current is applied though the device.[1] By scientific

definition, it is a solid-state device that controls current without the deficiency of having
heated filaments. How does a LED work? White LEDs ordinarily need 3.6 Volts of Direct
Current (DC) and use approximately 30 milliamps (mA) of current and has a power
dissipation of approximately 100 milliwatts (mW). The positive power is connected to one
side of the LED semiconductor through the anode and a whisker and the other side of the
semiconductor is attached to the top of the anvil or the negative power lead (cathode). It is
the chemical composition or makeup of the LED semiconductor that determines the color of
the light that the LED produces as well as the intensity level. The epoxy resin enclosure
allows most of the light to escape from the elements and protects the LED making it virtually
indestructible. Furthermore, a light-emitting diode does not have any moving parts, which
makes the device extremely resistant to damage due to vibration and shocks. These

Photodiode

A photodiode is a diode working in reverse polarization and having a window where the light

can enter and hit directly the pn junction. As in the case of the LED, the energy level of the
impurities has been chosen in order to allow electrons to jump from valence to conduction

Rectifier diodes

A rectifier is a dispositive that ideally transforms the AC input voltage into a DC voltage
(voltage is always positive or zero). These diodes have the largest ratings and sometime

can be quite big in volume. As a rule of thumb, the bigger the diode (more pn surface
junction available for heat dissipation), the higher the ratings.

Half-wave rectifier

A half-wave rectifier is composed of a single diode that connects an AC source to a load. In

figure 3 the load is represented by a resistor. The diode conducts on AC voltage only when

its anode is positive with respect to the cathode (i.e. greater than 0.7 V for a silicon diode).

The output has therefore only a positive component with an average value:

The output peak voltage is the AC source minus the voltage drop of the diode, that in most

cases can be neglected.

Full-wave rectifier

In half-wave rectifiers, half of the power provided by the source is not used. To solve this

problem, we have to use full-wave rectifiers. The minimum full-wave rectifier is composed

of two diodes, but it requires a center tapped transformer. Figure  shows a bridge rectifier,

composed of four diodes, that can use a “normal” transformer.

The AC current, according to its direction, flows either in the top or in the bottom part

of the bridge in each half-cycle. In the output voltage we will have a component for both

negative and positive parts of the input voltage. In both cases the current passes through

two forward-biased diodes in series, what produces a voltage drop of 1.4 V.

The average voltage of a full-wave rectifier is:

 Full wave rectifier. In this case the voltage drop, not shown in the graphic, will be

1.4 V because two diodes are cros.sed.