1. Solid-State Devices & Circuits 13. Emitter and Source Followers
ECE 342
Solid-State Devices & Circuits
13. Emitter and Source Followers
Jose E. Schutt-Aine
Electrical & Computer Engineering
University of Illinois

2. Emitter Follower

3. Emitter Follower High-Frequency
Exact analysis is too tedious  approximate

4. Emitter Follower

5. High-Frequency Analysis of Emitter Follower
sedr42021_0652a.jpg
leads to: 

6. High-Frequency Analysis of Emitter Follower

7. Source Follower with Active Load
Characteristics
Provides a buffer stage
M1 is amplifying stage
M2 is active load

8. Source Follower – Incremental Model

9. Source Follower – High Frequency
Since the source-to-substrate voltage for M1 varies, the body effect for that transistor must be included

10. Source Follower – High Frequency
Modified high-frequency model

11. Source Follower – Frequency Response

12. Emitter Follower with Active Load
(current mirror)
Emitter follower can be used to drive a low-impedance load

13. Emitter Follower with Active Load
Midband gain:

14. Emitter Follower with Active Load
AC Properties
Gain is less than 1 and near 1 for typical element values
Frequency response has one zero and two poles
Exact frequency response is difficult  Use SPICE
Output stage of NPN current mirror serves as high impedance load at emitter of Q1