Multi-stage transistor amplifiers

Question 1:

Don't just sit there! Build something!!

Learning to mathematically analyze circuits requires much study and

 practice. Typically, students practice by working through lots of sample

 problems and checking their answers against those provided by the

textbook or the instructor. While this is good, there is a much better way.
You will learn much more by actually building and analyzing real circuits,

 letting your test equipment provide the änswers" instead of a book or

another person. For successful circuit-building exercises, follow these steps:

1.

Carefully measure and record all component values prior to circuit

construction, choosing resistor values high enough to make

 damage to any active components unlikely.

2.

Draw the schematic diagram for the circuit to be analyzed.

3.

Carefully build this circuit on a breadboard or other convenient

 medium.

4.

Check the accuracy of the circuit's construction, following each

wire to each connection point, and verifying these elements

one-by-one on the diagram.

5.

Mathematically analyze the circuit, solving for all voltage and

current values.

6.

Carefully measure all voltages and currents, to verify the accuracy

 of your analysis.

7.

If there are any substantial errors (greater than a few percent),

carefully check your circuit's construction against the diagram,

then carefully re-calculate the values and re-measure.

When students are first learning about semiconductor devices, and are

 most likely to damage them by making improper connections in their

circuits, I recommend they experiment with large, high-wattage components

(1N4001 rectifying diodes, TO-220 or TO-3 case power transistors, etc.),

and using dry-cell battery power sources rather than a benchtop power

supply. This decreases the likelihood of component damage. As usual,

avoid very high and very low resistor values, to avoid measurement

 errors caused by meter "loading" (on the high end) and to avoid transisto

r burnout (on the low end). I recommend resistors between 1 kΩ and 100 kΩ.
One way you can save time and reduce the possibility of error is to begin

 with a very simple circuit and incrementally add components to increase

 its complexity after each analysis, rather than building a whole new circuit

 for each practice problem. Another time-saving technique is to re-use the

same componentsin a variety of different circuit configurations.

 This way, you won't have to measure any component's value more than once.

Reveal Answer

Let the electrons themselves give you the answers to your own

 "practice problems"!

Question 2:

Why is it common for amplifier circuits to use multiple stages of

transistors, rather than just one transistor (or two transistors in a

 push-pull circuit)? Describe some of the benefits of using multiple

transistor stages.

Reveal Answer

I'll let you research the answer(s) to this question on your own!

Notes:
A fairly simple question, but useful to discuss nevertheless.

Question 3:

Describe the function of each component in this two-stage amplifier circuit:

Also, be prepared to explain what the effect of any one component's failure

(either open or shorted) will have on the output signal.

Reveal Answer

•

R₁ = Q₁ biasing

•

R₂ = Q₁ biasing

•

R₃ = Q₁ load

•

R₄ = Q₁ stability (prevents thermal runaway)

•

R₅ = Q₂ biasing

•

R₆ = Q₂ biasing

•

R₇ = Q₂ load

•

R₈ = Q₂ stability (prevents thermal runaway)

•

C₁ = Input signal coupling to Q₁

•

C₂ = AC bypass for Q₁

•

C₃ = Coupling between amplifier stages

•

C₄ = AC bypass for Q₂

•

C₅ = Output signal coupling to load

•

Q₁ = First-stage amplification

•

Q₂ = Second-stage amplification

Notes:
The answers given in the Änswers" section are minimal: just enough to
help students who may be struggling with the concepts. During
discussion, I would expect more detail than these short phrases.
Be sure to challenge your students with hypothetical component
 failures in this circuit. Make sure they comprehend each component's
 function in this circuit, beyond memorizing a phrase!

Question 4:

In some applications where transistors must amplify very high currents,

 bipolar transistors are paralleled together so that their current ratings add:

However, if transistors are directly paralleled as shown, reliability problems

 may develop. A better way of "ganging" multiple transistors together is to

 connect a low-value swamping resistor to each emitter terminal:

Explain what purpose these resistors serve in a paralleled transistor network.

 And what exactly does ßwamping" mean, anyway?

Reveal Answer

Swamping is a design term, meaning to introduce a quantity or quantities

into a circuit such that any intrinsic differences between components become

insignificant in comparison. In this circuit, the swamping resistors help

ensure that the total controlled current is more evenly split between the

three transistors. Follow-up question: can you think of any disadvantages

 to using swamping resistors in high-power circuitry?

Notes:
I once had the misfortune of performing component-level repair on a large
power inverter (208 volt, three-phase) that used large "banks" of directly
paralleled bipolar transistors for the final switching elements. These inverters
had a bad habit of destroying transistors, and I noticed that invariably there
would be only one or two transistors out of about a dozen on each heat sink
rail that were blown - and I mean blown, holes blasted through the metal
TO-3 cases! - while the rest were perfectly fine. These transistor banks
 did not employ swamping resistors, and so the current distribution
between them was quite unbalanced.
In case students ask, you should let them know that swamping resistors
are not just used in transistor banks. Large rectifier diode banks
(where multiple diodes are paralleled) also benefit from swamping
resistors.
As for applications where swamping resistors are impractical, it is
possible to gain better reliability by using more transistors (or diodes)
 than necessary with an even current split. In other words, over-build
the circuit.

Question 5:

In some applications where transistors must amplify very high currents,

 bipolar transistors are paralleled together so that their current ratings

add. When this is done, it is a good idea to use swamping resistors at

 the transistor emitter connections to help ensure even balancing of currents:

However, if we use MOSFETs instead of BJTs, we do not have to use

swamping resistors:

Explain why MOSFETs do not require swamping resistors to help evenly

distribute current, while BJTs do.

Reveal Answer

The amount of controlling voltage varies with temperature for the BJT,

but not for the MOSFET.

Notes:
The answer given here is purposefully vague. Let your students do the
 necessary research! Tell them that manufacturers' application notes
are valuable sources of information for questions such as this.

Question 6:

The first amplifier circuit shown here is direct-coupled, while the second

 is capacitively coupled.

Which of these two designs would be more suitable for use in a DC

voltmeter circuit (amplifying a measured DC voltage)? What applications

 would the other amplifier design be suited for?

Reveal Answer

The direct-coupled amplifier circuit's bandwidth extends down to 0 Hz,

 unlike the other amplifier. This makes it suitable for DC signal amplification.

The capacitive-coupled amplifier circuit would be better suited for

 applications where AC signals are solely dealt with.
Follow-up question: in each of these amplifier circuits, identify the

 point at which the signal's phase becomes shifted by 180^o. In other

 words, show where the voltage signal becomes inverted, and then

inverted again, so that the output is in phase with the input.

Notes:
A good question to ask your students is, "What is bandwidth?" It is
important that your students understand the basic concept of "bandwidth"
, and what factors influence it in a circuit. Ask your students to suggest
 possible values (in microfarads) for the coupling capacitor in the second
 circuit, based on common resistor values (between 1 kΩ and 100 kΩ),
 and a modest audio frequency range(1 kHz to 20 kHz). No exact values
 are needed here, but it is important that they be able to make an approximate
 estimation of the necessary (minimum) capacitance, if for no other reason
 than to demonstrate their comprehension of the coupling capacitor's
 intended purpose.

Question 7:

One of the problems with capacitively-coupled amplifier circuits is poor

low-frequency response: as the input signal frequency decreases, all

capacitive reactances increase, leading to a decreased voltage gain.

One solution to this problem is the addition of a capacitor in the collector

current path of the initial transistor stage:

Explain how the presence of this "compensating" capacitor helps to

overcome the loss of gain normally experienced as a result of the

other capacitors in the circuit.

Reveal Answer

The additional capacitor's rising reactance at low frequencies boosts

 the gain of the first transistor stage by increasing the impedance

 from the first transistor's collector to the +V power supply rail.

Notes:
This technique is commonly used in video amplifier circuits, although
a complete video amplifier circuit would not be this crude (no peaking coils).