Review: BJT Amplifier Design. A BJT amplifier circuit should be designed toensure that the BJT operates in the active mode, allow the desired level of DC current to flow, andcouple to a small-signal input source and to an output
1. Lecture 11 OUTLINE
Review of BJT Amplifiers
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. 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.
Small-Signal Models for Independent Sources
5. Comparison of Amplifier Topologies Common Emitter
Large Av < 0
- Degraded by RE
- Degraded by RB/(b+1)
- 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
6. Common Emitter Stage
7. Common Base Stage
8. Emitter Follower
9. Ideal Current Source 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. 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. Boosting the Output Impedance
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.
17. Example: Determination of Voltage Gain
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.