1. Identifying amplifiers using a single transistor

Before we do detailed circuit analysis for a bunch of single-transistor amplifier types, it is useful to see the results that you get 90% of the time.

Overview of tables in Tourbook: Bipolar transistor amplifiers

Handout continues from day06_handout-orig.pdf which is selected pages from

Not all transistors are used for (linear) amplification! Sometimes it is simply an electronically-controlled switch.

Table 1. You should be able to identify the amplifier types for:
TA7642

  • T1

  • T2, T3

  • T4

  • T7

  • T10

Sony XDR F1HD

  • Q102

  • Q103

  • Q202

  • Q203
HF1 amateur radio transceiver

  • Q121

  • Q3

  • Q72, 73, 74

  • Q112

  • Q113

  • Q51

  • Q52
Yamaha PM1000

  • Q5

  • Q10

  • Q11

  • Q6

  • Q7

  • Q8, Q9

We will see all of these circuits several times each in the future.

2. DC bias conditions

aoe fig2 28 sim
Figure 1. Relying on β for stable bias conditions

Figure 1 first showed up in day05, let’s revisit the circuit. Then, the context was rapidly analyzing a BJT-containing circuit, which, in that case, meant finding a value for RB such that Vout = 7.5 V.

That analysis involved two unknowns: RB and β — remember that we assumed and set β = 100 for convenience.

2.1. Symbolic solution

Now it’s time to construct the circuit equations for VOUT, which will be a function of RB, RE, β, VBE, and VCC.

Note the capitalization of VOUT → we specifically want the DC solution to the circuit. Therefore, both Cs and Vs are effectively deleted from the analysis.

Solve the circuit symbolically:


IE =
(Ohm’s Law)


IB =
(definition of β, and Ohm’s Law)


VOUT =
(algebra)

3. Beta is a changeful constant

Open ON Semiconductor’s 2N2904 datasheet and consider the β situation. (hFE == β)

Details

Page 2: 2N3904 at 10 mA collector current Beta ranges 100—​300. The guaranteed minimum drops to 30 at the ends of the span of collector currents in the table. Also, Figure 15 indicates that hFE also varies from 50% to 150% of the room temperature value over the range -55 °C to 125 °C. This is a worst-case range of β ∈ 15—​450.

What are the conditions that apply for the numbers in the table?[1]

Use your symbolic circuit analysis from the last section with Figure 1.

How would you simulate the effects of varying β using LTspice or CircuitLab?

Is it better to use a simulator for seeing how β affects the circuit or better to see from the symbolic circuit analysis?

4. Common-emitter biasing

bjt beta test
Figure 2. The “but it works on paper!” circuit

If you thought the last circuit was sensitive to β, check this one out!

Consider Figure 2, set VCC = 10 V and find the conditions where the collector’s DC voltage is at mid-supply (e.g. 5 V).

List out the unknown quantities in this circuit first, then click to see if you missed any

  • RC

  • RB

  • β

  • VBE (or IS and VT)

  • also keep VCC symbolic

  • Use (symbolic) circuit analysis to find VC,

  • then fix a few numbers (VCC = 10 V, β = 100, and IC = 0.893 mA) and solve for the resistor values.[2]

  • Plot the how VC changes when β varies over 15—​450 with these resistor values.


VC =

1. Read the footnotes, you must.
2. Why the wacky current value? → because the resistor is a standard E12 value of 5.6 kΩ