Module 3: Amplifiers

Question 1

What are the three main properties of a transistor amplifier?

Answer 1

Input resistance, output resistance, and gain.

Question 2

Gain is the ratio of _____ divided by ______.

Answer 2

Output; input

Question 3

What is DC biasing, and why is it needed in a transistor amplifier situation?

Answer 3

For a transistor to amplify its input signal, it must be in the ACTIVE region (that is, above the cut-in voltage level, and below the minimum saturation voltage level). For maximum amplification, this ideal DC bias (or Q-point), is half-way across the load line curve.

Question 4

Why is small signal transistor ac analysis useful?

Answer 4

A transistor behaves very differently with a high-frequency ac input as opposed to a DC input.

Question 5

What are the three rules when converting a DC transistor circuit diagram to that of a small signal ac version?

Answer 5

1) any fixed voltages are regarded as ground 2) any capacitors may be shorted 3) the transistor is replaced by a current source (hfe) and its base-emitter resistance (hie)

Question 6

What is ‘h(ie)’, and what is its formula?

Answer 6

The input impedance of the equivalent ac transistor model.

h(ie) = 26e-3 / I(b)

Question 7

What is ‘h(fe)’?

Answer 7

The ‘forward current transfer ratio’ (dimensionless). It is approximately equal to the DC ‘beta’ or ‘gain’ value.

Question 8

What are the ‘h’ abbreviations in the small signal ac analysis model?

Answer 8

The “h” stands for “hybrid”, because the parameters are a mix of impedance, admittance, resistance, and dimensionless units.

Question 9

What is ‘gm’?

Answer 9

Transconductance g(m) = i(out) / v(in)

Question 10

What’s a coupling capacitor, and why is it important in transistor amplifiers?

Answer 10

To allow the coupling of signals between stages without the d.c. conditions of each stage affecting each other (capacitors are essentially short circuits with ac)

Question 11

Draw a Bode plot (i.e. frequency response curve) of a coupling capacitor. Label the gain/decade at appropriate points.

Answer 11

Question 12

What is the ‘break point’ of a frequency response graph of a coupling capacitor? How does this affect amplifier designs?

Answer 12

The point where the capacitor no longer changes its gain as its input frequency changes (typically at low frequencies). Amplifiers must use coupling capacitors in their STABLE state! Avoid the sloping part of the curve.

Question 13

What is an emitter-bypass capacitor, and why is it useful? Draw a diagram of: a) an example of it in a circuit; and b) its effective circuit at high and low frequencies

Answer 13

A capacitor across the emitter resistor to ground.

It gives a high gain in voltage (at high frequencyes), whilst retaining the advantages of having an emitter resistor (dc biasing stability) at low frequencies or dc conditions.

Question 14

What are some basic steps to determine what the role of a capacitor might be in an amplifier circuit?

Answer 14

Size: if in pF, it’s likely absorbing some noise; if in uF, it has a large effect on the circuit i.e. frequency response.

Location: if going to ground, it might be some for an ac short or noise absoprtion - if in series with signal, might be a coupling capacitor or high pass filter.

Question 15

What’s a differential amplifier? Draw a basic circuit diagram.

Answer 15

Has two inputs and amplifies the difference between those two inputs.

Question 16

Why does a differential amplifier have a the emitter-resistor in common for both transistors?

Answer 16

To try and keep the total current through Re constant.

Question 17

Define the Common Mode Rejection Ratio.

Answer 17

If a signal is applied equally to both inputs of an op amp, so that the differential input voltage is unaffected, the output should not be affected. In practice, changes in common mode voltage will produce changes in output. The op amp common-mode rejection ratio (CMRR) is the ratio of the common-mode gain to differential-mode gain.

Question 18

What’s the formula for the CMRR? Draw a diagram showing the difference between the two types of gain.

Answer 18

CMMR = Difference gain (hopefully high) / common gain (hopefully low)

Question 19

What’s a typical CMRR figure for an op-amp?

Answer 19

100dB

Question 20

What would the gain be if one were to take an output between ONE collector and ground, and why?

Answer 20

Half that of the full gain

Due to Re, the current is halved between both transistors. Take out one transistor, and you have half the gain!

Question 21

What is the purpose of this circuit?

Answer 21

Constant current source

V(base) = constant

Question 22

What is the conceptual difference between a power amplifier and a regular transistor amplifier?

Answer 22

A transistor amplifier’s main purpose is to amplify voltage, and are very inefficient - generally ~11%.

Power amplifiers need to output lots of power (generally by increasing the size of the output current). They are more efficient (~20% for class A).

Question 23

Why are transistor amplifiers so inefficient? Explain.

Answer 23

A biased transistor needs a low of power supplied to it just to run at its bias point.

An rms signal requires very little power to amplify.

Efficiency = in / out, hence very inefficient.

Question 24

Where does the lost energy go in a power amplifier?

Answer 24

~25% = heat in transistor

~50% = DC heat in resistor

~25% = AC heat in resistor

Question 25

State the type of amplifier, as well as its gain equation.

Answer 25

Inverting op-amp;

Gain = R(f) / R(in)

Question 26

State the type of amplifier, as well as its gain equation.

Answer 26

Non-inverting op-amp;

Gain = {R1 + R(f)} / R1

Question 27

State the type of amplifier.

Answer 27

Differential op-amp

Question 28

What does this circuit do (i.e. what is it called?)

Answer 28

Integrating op-amp

Question 29

What does this circuit do (i.e. what is it called?)

Answer 29

Summing op-amp

Question 30

What are some losses in an actual op-amp?

Answer 30

• transistor bias current - transistors inside op-amp need a DC bias.
• input offset error - CMRR

Question 31

How does one mitigate the transistor bias current in an op-amp?

Answer 31

Compensating resistor

Question 32

What is the ‘corner (aka 3dB) frequency’ in the context of a transistor amplifier? What is its formula?

Answer 32

The point where an amplifier switches from its 20dB/decade steady gain to a stable ‘mid-band’ gain (the area in which we want to use it).

f(corner) = 1/2pi * R(in) * C(in)

Question 33

What is the formula for the input resistance of your classic transistor amplifier?

Answer 33

where R(b) = R1 // R2