Common Collector Amplifier - Electrical Engineering (EE) PDF Download

Common Collector Amplifier: 

If a high impedance source is connected to low impedance amplifier then most of the signal is dropped across the internal impedance of the source. To avoid this problem common collector amplifier is used in between source and CE amplifier. It increases the input impedance of the CE amplifier without significant change in input voltage.

Fig. 1, shows a common collector (CC) amplifier. Since there is no resistance in collector circuit, therefore collector is ac grounded. It is also called grounded collector amplifier. When input source drives the base, output appears across emitter resistor. A CC amplifier is like a heavily swamped CE amplifier with a collector resistor shorted and output taken across emitter resistor.
v_out = v_in - V_BE

Therefore, this circuit is also called emitter follower, because V_BE is very small. As v_in increases, v_out increases.

If v_in is 2V, v_out = 1.3V

If v_in is 3V, v_out = 2.3V.

Since v_out follows exactly the v_in therefore, there is no phase inversion between input and output.

The output circuit voltage equation is given by-

V_CE = V_CC – I_E R_E

Since I_E = I_C

 I_C = (V_CC – V_CE ) / R_E

This is the equation of dc load line. The dc load line is shown in Fig. 1.

Common Collector Amplifier
Voltage gain:

Fig. 2, shows an emitter follower driven by a small ac voltage. The input is applied at the base of transistor and output is taken across the emitter resistor. Fig. 3, shows the ac equivalent circuit of the amplifier. The emitter is replaced by ac resistance r'_e.

The ac output voltage is given by

v_out = R_E i_e

and, v_in = i_e (R_E + r'_e )Therefore, A = R_E / ( R_E +r'_e )       

Since r'_e << R_E

Av=1. (approx)

Therefore, it is a unity gain amplifier. The practical emitter follower circuit is shown in Fig. 4

The ac source (v_S) with a series resistance R_S drives the transistor base. Because of the biasing resistor and input impedance of the base, some of the ac signal is lost across the source resistor. The ac equivalent circuit is shown in Fig. 5.

The input impedance at the base is given by

The total input impedance of an emitter follower includes biasing resistors in parallel with input impedance of the base.

z_in = R₁ || R₂||  (r'_e + R_E)

Since  R_E is very large as compared to R₁ and R₂.

Thus,      z_in ≈ R₁ || R₂

Therefore input impedance is very high.

Applying Thevenin's theorem to the base circuit of Fig. 5, it becomes a source v_in and a series resistance (R₁ || R₂ || R_S ) as shown in Fig. 6.

Common Collector Amplifier

 Example 1: 

Find the Q-point of the emitter follower circuit of fig. 7 with R₁ = 10 KΩ and R₂ = 20 KΩ. Assume the transistor has a β of 100 and input capacitor C is very-very large.

Solution: 

We first find the Thevenin's equivalent of the base bias circuitry.

R_B = R₁ || R₂ = 6.67 K Ω

From the bias equation we have

Example - 2 

Find the output voltage swing of the circuit of fig. 7.

Solution: 

The Q-Point location has already been calculated in Example-1. We found that the quiescent collector current is 4.95 mA.

The Output voltage swing = 2 . I_C peak . (R_E || R_Load) = 2(4.95 x 10^-3) (300) = 2.97V

This is less than the maximum possible output swing. Continuing the analysis,

V_CEQ = V_CC – I_CQ R_E = 9.03 V

V_CC = V_CEQ + I_CQ (R_E || R_Load ) = 10.5 V

The load lines for this problem are shown in Fig. 8.

                                  fig. 8