1. Multi-stage Amplifiers
Single stage limitations
Multi-stage amplifiers
Feedback

2. Single Stage Amplifier Limitations
High gain Þ high gm and RC
High rIN Þ low gm
Low rOUT Þ low RC

3. Simple 2 Stage Amplifier
To increase the gain, use two amplifiers instead of one.
Gain is not just the product of the individual gains though
Input and output impedances must be accounted for

4. Equivalent Circuit

5. Coupling
In the simple 2 stage design, the stages are linked by coupling capacitors
This means that the biasing networks for each stage can be designed independently
It also means that d.c. operation is impossible
To do this:
The capacitors must go
Each stage supplies the bias current for the following stage

6. Differential Input 2 Stage Amplifier
BAD DESIGN!
Good points
Q1 and Q2 form a standard differential amplifier
RC1 forms the collector resistor for Q2 and provides base bias current for Q3
Q3 forms a common-emitter amplifier
Bad point
vOUT can’t go below 0V
IB3

7. Using PNP Transistors
0.5 V
0.5 V
Analysis of PNP based circuit is almost identical to NPN
Only real difference is that the currents in a PNP transistor all flow in the opposite direction

8. Practical Two Stage Amplifier
Q3 and RC2 form a common emitter amplifier again
Base bias current for Q3 is provided by Q2 collector
Output voltage can vary almost right up to ±VS
Multi-stage amplifiers are usually made up from alternating NPN and PNP stages

9. Example – 1. Quiescent Conditions
With vIN = 0, we know that:
0.5 V

10. Example – 2. Differential Stage

11. Example – 3. Common Emitter

12. Example – 4. Overall Response

13. Predictability and Distortion
Problems
IC3
IC3 depends on current gain of Q3
This is unpredictable
Therefore, gain and quiescent vOUT are also unpredictable
Linearity
To get near the maximum possible output, amplifier goes into the non-linear region

14. Using Feedback
In exactly the same way as with the operational amplifier, non-ideal properties of the multi-stage amplifier can be compensated for using negative feedback.
E.g.
Gain can be made predictable
Quiescent output level can be corrected (offset voltage removed)
Frequency response extended (constant gain-bandwidth product)
Explore these ideas during lab session 3.

15. Transistors vs. Op-Amps
Operational Amplifiers
Very simple to use – easy calculations
Near ideal properties
Transistors
Better frequency response
Lower noise

16. Today’s Summary
Multi-stage amplifiers can be considered as many separate amplifiers connected together
Use of the equivalent circuits for the amplifiers eases analysis
Direct coupling enables low-frequency use
Negative feedback can improve linearity and bandwidth

17. 2B1 Analogue Electronics Summary
Practical Op-Amp Design
How negative feedback works
GB Product, Slew Rate, Saturation etc.
Non-Linear Op-Amp Applications
Comparators, Schmitt Triggers, Precision Rectifiers
Transistor Amplifiers
Common Emitter Amplifier
Differential Amplifier
Multi-stage amplifiers
Design and analysis of all the above