COMMON-COLLECTOR AMPLIFIER

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1 COMMON-COLLECTOR AMPLIFIER
SMALL-SIGNAL VOLTAGE GAIN INPUT AND OUTPUT IMPEDANCES SMALL-SIGNAL CURRENT GAIN

2 C-C Amplifier Collector connected directly to Vcc
Output signal taken at emitter with respect to ground Emitter-follower circuit

3 Small-signal equivalent circuit of emitter-follower
Vcc or collector terminal is at signal ground. o/p resistance, ro parallel with dependent current source.

4 Another small-signal equivalent circuit of emitter follower
Assume Cc is short-circuited. ro parallel with dependent current source and collector terminal is at ground. Rearrange the previous cct so that all signal grounds connected are at same point, we will get..

5 C-C Amplifier equations
We see that Output voltage is written as Apply KVL around base-emitter loop Or We can also write Where Combine all above equations, the small-signal voltage gain is:

6 Input impedance Input resistance looking into the base, Rib is given by: Since Ie is (1+β) times Ib, the effective impedance in emitter is multiplied by (1+β). This multiplication is known as resistance reflection rule. Input resistance at the base, Rib is: rπ + [effective resistance in emitter x (1+β)]

7 Output resistance To find o/p resistance, assume input signal source, Vs is ideal and Rs=non-zero. Refer to below figure, we can determine o/p resistance looking back into o/p terminals. Set independent voltage source, Vs=0 (short circuit).

8 Output resistance A test voltage, Vx is applied to o/p terminal and produce test current, Ix. The control voltage, Vπ is not zero, but is a function of test voltage. The output resistance, Ro is given by: Summing current at the o/p node, we have: Control voltage in term of test voltage using voltage divider:

9 Output resistance Substitute the above eq and rearrange leads to:
Note that, gmrπ=β, we find that: Or

10 Things to remember.. DEPENDENT SOURCE
IS A SOURCE WHICH CAN ONLY GENERATE SIGNALS IF THERE IS ANOTHER SOURCE GIVING A SIGNAL INDEPENDENT SOURCE GENERATES ITS OWN SIGNAL

11 Small-signal equivalent circuit - small-signal current gain

12 Small-signal current gain, Ai
Determine current gain using input resistance and concept of current divider. Small-signal current gain is defined as: Using current divider, we write base current in term of input current:

13 Small-signal current gain
Since gmVπ=βIb, then Write Ie in term of Io produces Combine all equations yield to: If R1||R2 >> Rib and ro>>RE, then Current gain >> 1 (unity). Voltage gain < 1. Thus, C-C circuit produces small-signal power gain

14 COMMON-BASE AMPLIFIER
SMALL-SIGNAL VOLTAGE GAIN INPUT AND OUTPUT IMPEDANCES SMALL-SIGNAL CURRENT GAIN

15 Common-base circuit Use hybrid-π equivalent circuit. Dc analysis is the same as C-E circuit. Base is at signal ground and input signal is applied to emitter. The output signal is measured at collector terminal. Assume load is connected to output through coupling capacitor, Cc2.

16 C-B Small-signal equivalent circuit
Hybrid-π model for npn transistor with ro=∞ Small-signal equivalent circuit for C-B

17 Small-signal voltage gain, Av
Rearrange previous 2 equations, small-signal voltage gain is: As Rs  0, voltage gain becomes: Output voltage is given by: Apply KCL at emitter node, Since β=gmrπ, above eq. can be written as: Then,

18 Small-signal current gain, Ai
Ai = Io / Ii . Apply KCL at emitter node, Solving for Vπ, we obtain: The load current is: Combine all above equations, small-signal current gain is: If RE ∞ and RL 0, current gain equation becomes For C-B circuit, voltage gain >> 1 and current gain < 1. So, we get small power gain. Application of C-B cct take advantage of i/p and o/p resistance characteristic.

19 C-B Input impedance Rie
Small-signal equivalent cct looking into emitter. The control voltage reverse the direction of dependent current source. The input resistance looking into the emitter is: + V - r Rie gmV RC RL Ii

20 Input impedance Apply KCL at the input, Thus,
The resistance looking into the emitter with base grounded -> re and is quite small. When input signal is a current source, a small input resistance is desirable.

21 Output impedance Ro C-B circuit for output resistance calculations -
V + r Ro gmV RC amplifier RS RE Vx Ix E

22 Output impedance Independent source, vs set = 0.
KCL equation at the emitter, Since vs =0, Vπ=0 and so do gmVπ = 0. The output resistance looking into the o/p terminals is:

23 Comparison Voltage gain Current gain Input resistance
Configuration Voltage gain Current gain Input resistance Output resistance Common emitter Av > 1 Ai > 1 Moderate Moderate to high Emitter follower Av ≈ 1 High Low Common base Ai ≈ 1