**Common Collector Circuit Analysis:**

In the Common Collector Circuit Analysis (CC) shown in Fig. 6-28 the external load (R_{L}) is capacitor-coupled to the transistor emitter terminal. The circuit uses voltage divider bias to derive the transistor base voltage (V_{B}) from the supply. The transistor collector terminal is directly connected to V_{CC,} no collector resistor is used. The circuit output voltage is developed across the emitter resistor (R_{E}), and there is no bypass capacitor.

To understand the operation of a Common Collector Circuit Analysis, note that V_{B} is a constant quantity, and that V_{E} = V_{B} – V_{BE}. When a signal is applied via C_{1} to the transistor base, V_{B} increases and decreases as the signal goes positive and negative. If the signal voltage (v_{s}) increases to +0.5 V, V_{B} is increased by 0.5 V. Also, V_{E} increases by 0.5 V, because V_{BE} remains substantially constant, and V_{E} = V_{B} – V_{BE}. The change in V_{E} is coupled via C_{2} to give an ac output voltage (v_{o} = 0.5 V), (see the waveforms in Fig. 6-29). Similarly, when v_{s} decreases to -0.5 V, both V_{B} and V_{E} decrease by 0.5 V, giving v_{o} = -0.5 V.

It is seen that the ac output voltage from a CC circuit is essentially the same as the input voltage; there is no voltage gain or phase shift. Thus, the Common Collector Circuit Analysis can be said to have a voltage gain of 1. (Actually, the output voltage can be shown to be slightly smaller than the input because of a very small change in V_{BE}.)

The fact that the CC output voltage follows the changes in signal voltage gives the circuit its other name: emitter follower.

**h-Parameter Equivalent Circuit:**

As for a CE circuit, the power supply and capacitors must be replaced with short-circuits to study the CC ac performance. This gives the CC ac equivalent circuit in Fig. 6-30(a). The input terminals of the ac equivalent circuit are seen to be the transistor base and collector, and the output terminals are the emitter and collector. Because the collector terminal is common to both input and output, the circuit configuration is named Common Collector Circuit Analysis.

The CC h-parameter circuit is now drawn by substituting the transistor h-parameter model into the ac equivalent circuit, to give the circuit in Fig. 6-30(b). The current directions and voltage polarities indicated in Fig. 6-30(b) are, once again, those that are produced by a positive-going signal voltage. It should be noted that h_{rc} = 1 for a CC circuit; all of v_{o} is fed back to the input. So, unlike the case of a CE circuit, the feedback generator cannot be omitted in the equivalent circuit of a CE amplifier.

**Input Impedance:**

The input impedance for the Common Collector Circuit Analysis is determined by first writing an equation for the input voltage. Referring to Fig. 6-30 and Fig. 6-31,

Equation 6-23 is similar to the equation for the transistor input impedance in a CE circuit with an unbypassed emitter resistor (Eq. 6-20), except that R_{L} is now in parallel with R_{E}. The circuit input impedance is again given by Eq. 6-12,

Using Eq. 6-23, the input impedance at the transistor base in a CC circuit can be quickly estimated. A circuit with R_{E} = 1 kΩ and h_{fe} = 100, has Z_{b} ≈ 100 kΩ if R_{L} ≫R_{E}.

**Output Impedance:**

As already discussed, the at voltage at the output of a CC circuit is all fed back to the input. This fact is used in determining the output impedance (Z_{e}) at the emitter terminal. The signal voltage is assumed to be zero, and v_{o} is used to calculate I_{e}. With v_{s} = 0, I_{b} is produced by the fed back voltage (h_{rc} v_{o} =v_{o}), [see Fig. 6-32(a)].

Note that the output impedance at the emitter terminal is

It is interesting to compare this to the base input impedance (Eq. 6-23), which is,

Equation 6-24 gives the device output impedance. (Actually, 1/ h_{oc} should be included, but it has a negligible effect on Z_{e}.) The circuit output impedance also involves R_{E}, [see Fig. 6-32(b)].

R_{E} is usually much larger than Z_{e}, so that Z_{o} ≈ Z_{e}.

**Voltage Gain:**

As already explained, the ac output voltage from a CC circuit is almost exactly equal to the input voltage. However, the precise equation for A_{v} is easily derived by considering the ac output and input voltages as derived from Fig, 6-30(b).

this reduces to,

Because r′_{e} ≈ h_{ib}, Eq. 6-26 can be used in situations where the device parameters are unknown. Usually, R_{E} || R_{L} is so much larger than h_{ib} that the CC circuit voltage gain is simply taken as,

**Summary of CC Circuit Performance:**

A Common Collector Circuit Analysis has a voltage gain of 1, no phase shift between input and output, high input impedance, and low output impedance. Because of its high Z_{i}, low Z_{o}, and unity gain, the CC circuit is normally used as a buffer amplifier, placed between a high impedance signal source and a low impedance load.