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Summary of Chapter 6 BJT Amplifiers

6-1: Amplifier Operation

The coupling capacitors: block dc and thus prevent the internal source resistance, Rs, and the load resistance, RL, from changing the dc bias voltages at the base and collector. Operation: Vb vary sinusoidally on VBQ , Ib vary sinusoidally on IBQIC varies sinusoidally on ICQ (in-phase), Vce varies sinusoidally with VCE (out of phase) .

6-2 Transistor AC models A transistor model circuit uses various internal transistor parameters (resistance or r parameters and hybrid h parameters) to represent its operation.

r- parameters h- parameter s(datasheet)

Description

αac hfb Ic /Ie

βac hfe Ic /Ib

r'e hre / hoe ac emitter resistance r'e = IE / 25 mV

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6-3: The Common-Emitter Amplifier (CE)

1. The emitter is the common terminal (ground) for the ac signal. 2. High voltage gain 3. High current gain

Notes: C1, C3 are coupling capacitors and C2 is bypass capacitor. There is no signal at the emitter because C2 shorts the emitter to ground at the signal frequency.

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Effect of the emitter Bypass capacitor on the voltage gain (Stability of Gain ) The value of bypass capacitor must be large enough so that (10 Xc <= RE).

Notice that r'e is dependent of temperature and thus if RE >> r'e Gain is stable

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Swamping to stabilize the voltage gain (RE: partially bypassed) to reduce r'e effect on gain)

Example (6-8): For the amplifier in Figure 6–20, (a) Determine the dc collector voltage. (b) Determine the ac collector voltage. (c) Draw the total collector voltage waveform and the total output voltage waveform.

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(a) Determine the dc bias values using the dc equivalent circuit in

(b) The ac analysis is based on the ac equivalent circuit

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(c) The total collector voltage is the signal voltage of 84.5 mV rms riding on a dc level of 5.02 V: Max Vc( p) = VC + 1.414 Vc =5.02 V + (84.5 mV)(1.414) = 5.13 V

Min Vc (p) = VC - 1.414 Vc =5.02 V - (84.5 mV) (1.414) = 4.9 V

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Current Gain and Power Gain The current gain from base to collector is βac = Ic / Ib or however, the overall current gain of

the common-emitter amplifier is

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6-4: The Common-Collector Amplifier (CC) Usually referred to as an Emitter – Follower (EF)

Av ≈ 1

No phase inversion

Main advantages: (1) high input resistance (2) high current gain

Darlington Pair: Used to boost the input resistance which is limited by βac. The collectors of two transistors are connected, and the emitter of the first drives the base of the second. This configuration achieves βac multiplication.

An Application The emitter-follower is often used as an interface between a circuit with a high output resistance and a low-resistance load. In such an application, the emitter-follower is called a buffer.

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Application Example: An emitter-follower using a Darlington pair can be used to interface the amplifier and The speaker as shown:

6-5: The Common-Base Amplifier (CB)

High voltage gain

maximum current gain of 1

low input resistance appropriate type for certain applications where sources tend to have very low-resistance outputs

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