1. Lecture 11 ANNOUNCEMENTS OUTLINE Review of BJT Amplifiers
Prob. 5 of Pre-Lab #5 has been clarified. (Download new version.)
HW#6 has been posted.
Review session: 3-5PM Friday (10/5) in 306 Soda (HP Auditorium)
Midterm #1 (Thursday 10/11):
Material of Lectures 1-10 (HW# 2-6; Chapters 2, 4, 5)
2 pgs of notes (double-sided, 8.5”×11”), calculator allowed
OUTLINE
Review of BJT Amplifiers
Cascode Stage
Reading: Chapter 9.1

2. Review: BJT Amplifier Design
A BJT amplifier circuit should be designed to
ensure that the BJT operates in the active mode,
allow the desired level of DC current to flow, and
couple to a small-signal input source and to an output “load”.
Proper “DC biasing” is required!
(DC analysis using large-signal BJT model)
Key amplifier parameters:
(AC analysis using small-signal BJT model)
Voltage gain Av  vout/vin
Input resistance Rin  resistance seen between the input node and ground (with output terminal floating)
Output resistance Rout  resistance seen between the output node and ground (with input terminal grounded)

3. Large-Signal vs. Small-Signal Models
The large-signal model is used to determine the DC operating point (VBE, VCE, IB, IC) of the BJT.
The small-signal model is used to determine how the output responds to an input signal.

4. Small-Signal Models for Independent Sources
The voltage across an independent voltage source does not vary with time. (Its small-signal voltage is always zero.)
It is regarded as a short circuit for small-signal analysis.
The current through an independent current source does not vary with time. (Its small-signal current is always zero.)
It is regarded as an open circuit for small-signal analysis.
Large-Signal
Model
Small-Signal
Model

5. Comparison of Amplifier Topologies
Common Emitter
Large Av < 0
- Degraded by RE
- Degraded by RB/(b+1)
Moderate Rin
- Increased by RB
- Increased by RE(b+1)
Rout  RC
ro degrades Av, Rout
but impedance seen
looking into the collector
can be “boosted” by
emitter degeneration
Common Base
Large Av > 0
-Degraded by RE and RS
- Degraded by RB/(b+1)
Small Rin
- Increased by RB/(b+1)
- Decreased by RE
Rout  RC
ro degrades Av, Rout
but impedance seen
looking into the collector
can be “boosted” by
emitter degeneration
Emitter Follower
0 < Av ≤ 1
- Degraded by RB/(b+1)
Large Rin
(due to RE(b+1))
Small Rout
- Effect of source
impedance is
reduced by b+1
- Decreased by RE
ro decreases Av, Rin, and Rout

6. Common Emitter Stage

7. Common Base Stage

8. Emitter Follower

9. Ideal Current Source
Circuit Symbol
I-V Characteristic
Equivalent Circuit
An ideal current source has infinite output impedance.
How can we increase the output impedance of a BJT that
is used as a current source?

10. Boosting the Output Impedance
Recall that emitter degeneration boosts the impedance seen looking into the collector.
This improves the gain of the CE or CB amplifier. However, headroom is reduced.

11. Cascode Stage
In order to relax the trade-off between output impedance and voltage headroom, we can use a transistor instead of a degeneration resistor:
VCE for Q2 can be as low as ~0.4V (“soft saturation”)

12. Maximum Cascode Output Impedance
The maximum output impedance of a cascode is limited by r1.

13. PNP Cascode Stage

14. False Cascodes
When the emitter of Q1 is connected to the emitter of Q2, it’s not a cascode since Q2 is a diode-connected device instead of a current source.

15. Short-Circuit Transconductance
The short-circuit transconductance of a circuit is a measure of its strength in converting an input voltage signal into an output current signal.

16. Voltage Gain of a Linear Circuit
By representing a linear circuit with its Norton equivalent, the relationship between Vout and Vin can be expressed by the product of Gm and Rout.
Norton Equivalent Circuit
Computation of
short-circuit
output current:

17. Example: Determination of Voltage Gain
Determination of Gm
Determination of Rout

18. Comparison of CE and Cascode Stages
Since the output impedance of the cascode is higher than that of a CE stage, its voltage gain is also higher.

19. Voltage Gain of Cascode Amplifier
Since rO is much larger than 1/gm, most of IC,Q1 flows into diode-connected Q2. Using Rout as before, AV is easily calculated.

20. Practical Cascode Stage
No current source is ideal; the output impedance is finite.

21. Improved Cascode Stage
In order to preserve the high output impedance, a cascode PNP current source is used.