1. Solid-State Devices & Circuits
ECE 442
Solid-State Devices & Circuits
9. BJT Amplifiers
Jose E. Schutt-Aine
Electrical & Computer Engineering
University of Illinois

2. Small-Signal Model What is a small-signal incremental model?
Equivalent circuit that only accounts for signal level fluctuations about the DC bias operating points
Fluctuations are assumed to be small enough so as not to drive the devices out of the proper range of operation
Assumed to be linear
Derives from superposition principle

3. Hybrid-p Incremental Model for BJTs
rp: input resistance looking into the base
rx: parasitic series resistance looking into base – ohmic base resistance
gm: BJT transconductance
ro=rce: output collector resistance related to the Early effect

4. Hybrid-p Parameters
Can show that

5. Common Emitter Configuration
The emitter current IE can be approximated as:
An incremental conductance ge can be defined as
From which we get:
Emitter resistance

6. Common Emitter (CE) Amplifier
Bias: Choose R1 & R2 to set VBVE is then set. Choose RE to set IE~IC. Quiescent point of Vout will be determined by RC. Emitter is an AC short.

7. Incremental Model for CE Amplifier
Hybrid-p model (ignoring rx)

8. CE Amplifier
gain from base to collector

9. CE Amplifier
Open-circuit voltage gain:

10. CE Amplifier Output Impedance
It can be seen that if Rsig >> rp, the gain will be highly dependent on b. This is not good because of b variations

11. CE with External Resistors

12. CE with External Resistors

13. CE with External Resistors
RE and RB degrade the gain

14. CE with External Resistors
The gain can be written as:

15. Example
Given VBEON=0.6V, find the gain for the circuit shown

16. Example (Cont’)
AC analysis: RE2 is shorted and RE=RE1=100W. Since b is not known, use:

17. Emitter Follower
circuit
Incremental model

18. Emitter Follower
Emitter follower has unity voltage gain

19. Emitter Follower – Input Impedance

20. Emitter Follower – Output Impedance

21. Output Impedance (cont’)
If we neglect RB

22. Common Base Configuration

23. Common Base Configuration

24. BJT Topologies - SummaryCE CB EF 1
Avo
Rin
Rout