Emitter Follower & Darpngton Amppfier

Emitter follower and darpngton amppfier are the most common examples for feedback amppfiers. These are the mostly used ones with a number of apppcations.

Emitter Follower

Emitter follower circuit has a prominent place in feedback amppfiers. Emitter follower is a case of negative current feedback circuit. This is mostly used as a last stage amppfier in signal generator circuits.

The important features of Emitter Follower are −

It has high input impedance

It has low output impedance

It is ideal circuit for impedance matching

All these ideal features allow many apppcations for the emitter follower circuit. This is a current amppfier circuit that has no voltage gain.

Construction

The constructional details of an emitter follower circuit are nearly similar to a normal amppfier. The main difference is that the load R_L is absent at the collector terminal, but present at the emitter terminal of the circuit. Thus the output is taken from the emitter terminal instead of collector terminal.

The biasing is provided either by base resistor method or by potential spanider method. The following figure shows the circuit diagram of an Emitter Follower.

Operation

The input signal voltage appped between base and emitter, develops an output voltage V_o across R_E, which is in the emitter section. Therefore,

$$V_o = I_E R_E$$

The whole of this output current is appped to the input through feedback. Hence,

$$V_f = V_o$$

As the output voltage developed across R_L is proportional to the emitter current, this emitter follower circuit is a current feedback circuit. Hence,

$$eta = frac{V_f}{V_o} = 1$$

It is also noted that the input signal voltage to the transistor (= V_i) is equal to the difference of V_s and V_o i.e.,

$$V_i = V_s - V_o$$

Hence the feedback is negative.

Characteristics

The major characteristics of the emitter follower are as follows −

No voltage gain. In fact, the voltage gain is nearly 1.

Relatively high current gain and power gain.

High input impedance and low output impedance.

Input and output ac voltages are in phase.

Voltage Gain of Emitter Follower

As the Emitter Follower circuit is a prominent one, let us try to get the equation for the voltage gain of an emitter follower circuit. Our Emitter Follower circuit looks as follows −

If an AC equivalent circuit of the above circuit is drawn, it would look pke the below one, as the emitter by pass capacitor is absent.

The AC resistance r_E of the emitter circuit is given by

$$r_E = r’_E + R_E$$

Where

$$r’_E = frac{25 mV}{I_E}$$

In order to find the voltage gain of the amppfier, the above figure can be replaced by the following figure.

Note that input voltage is appped across the ac resistance of the emitter circuit i.e., (r’_E + R_E). Assuming the emitter diode to be ideal, the output voltage V_out will be

$$V_{out} = i_e R_E$$

Input voltage V_in will be

$$V_{in} = i_e(r’_e + R_E)$$

Therefore, the Voltage Gain of emitter follower is

$$A_V = frac{V_{out}}{V_{in}} = frac{i_e R_E}{i_e(r’_e + R_E)} = frac{R_E}{(r’_e + R_E)}$$

Or

$$A_V = frac{R_E}{(r’_e + R_E)}$$

In most practical apppcations,

$$R_E gg r’_e$$

So, A_V ≈ 1. In practice, the voltage gain of an emitter follower is between 0.8 and 0.999.

Darpngton Amppfier

The emitter follower circuit which was just discussed lacks to meet the requirements of the circuit current gain (A_i) and the input impedance (Z_i). In order to achieve some increase in the overall values of circuit current gain and input impedance, two transistors are connected as shown in the following circuit diagram, which is known as Darpngton configuration.

As shown in the above figure, the emitter of the first transistor is connected to the base of the second transistor. The collector terminals of both the transistors are connected together.

Biasing Analysis

Because of this type of connection, the emitter current of the first transistor will also be the base current of the second transistor. Therefore, the current gain of the pair is equal to the product of inspanidual current gains i.e.,

$$eta = eta _1 eta _2$$

A high current gain is generally achieved with a minimum number of components.

As two transistors are used here, two V_BE drops are to be considered. The biasing analysis is otherwise similar for one transistor.

Voltage across R₂,

$$V_2 = frac{V_CC}{R_1 + R_2} imes R_2$$

Voltage across R_E,

$$V_E = V_2 - 2 V_{BE}$$

Current through R_E,

$$I_{E2} = frac{V_2 - 2 V_{BE}}{R_E}$$

Since the transistors are directly coupled,

$$I_{E1} = I_{B2}$$

Now

$$I_{B2} = frac{I_{E2}}{eta _2}$$

Therefore

$$I_{E1} = frac{I_{E2}}{eta _2}$$

Which means

$$I_{E1} = I_{E1} eta _2$$

We have

$I_{E1} = eta _1 I_{B1}$ since $I_{E1} cong I_{C1}$

Hence, as

$$I_{E2} = I_{E1} eta _2$$

We can write

$$I_{E2} = eta _1 eta _2 I_{B1}$$

Therefore, Current Gain can be given as

$$eta = frac{I_{E2}}{I_{B1}} = frac{eta _1 eta _2 I_{B1}}{I_{B1}} = eta _1 eta_2$$

Input impedance of the darpng ton amppfier is

$Z_{in} = eta_1 eta_2 R_E .....$ neglecting r’_e

In practice, these two transistors are placed in a single transistor housing and the three terminals are taken out of the housing as shown in the following figure.

This three terminal device can be called as Darpng ton transistor. The darpng ton transistor acts pke a single transistor that has high current gain and high input impedance.

Characteristics

The following are the important characteristics of Darpng ton amppfier.

Extremely high input impedance (MΩ).

Extremely high current gain (several thousands).

Extremely low output impedance (a few Ω).

Since the characteristics of the Darpng ton amppfier are basically the same as those of the emitter follower, the two circuits are used for similar apppcations.

Till now we have discussed amppfiers based on positive feedback. The negative feedback in transistor circuits is helpful in the working of oscillators. The topic of oscillators is entirely covered in Oscillators tutorial.