Design an audio amplifier using the circuit shown. Assuming the collector current Icof the amplifier is around 1 mA, determine the values of R1, R2, Rcand Re.
Solution
The single-stage transistor amplifier shown in the diagram is a circuit that is often used to provide an inverting amplifier with moderate voltage gain.
Although the calculation is beyond the scope of this book, if Rcand Reare relatively large (>1 kΩ) and Icis relatively large (≥1 mA) then the voltage gain for a time-varying signal can be approximated by:
Av= ~
Designing such a transistor amplifier circuit is not a very precise art, and several engineering ‘rules of thumb’ are used. These rules are:
1 Vc~
2 Ve~ 3 R2≤10Re
The first two rules ensure that the transistor is biased near the middle of its output range (so we can allow for large ∆voutsignals with minimum distortion) and that the circuit is stable with a reasonable voltage gain.
The third rule keeps the base current (Ib) small compared with the current flowing through the R1: R2voltage divider, which ensures that the amplifier does not distort the signal that is to be amplified.
Applying rule 1, we see that Vcshould be ~6 V.
If we choose Ic~ 1 mA (a normal value for a typical transistor signal amplifier), then from Vc=Vcc−IcRc:
Rc=
=
=6 kΩ Applying rule 2:
Ve~
=IeRc
≈IcRe(remember that Ie≈Icbecause Ib<< Ieor Ic)
Hence Re= =1.2 kΩ
To calculate Vb: Vb=Vbe+Ve
=0.7 +1.2
=1.9 V Appling rule 3:
R2≤10Re
≤10 ×1.2 kΩ 1.2 1 ×10−3 Vcc
10 12 −6 1 ×10−3 Vcc−Vc
Ic Vcc
10 Vcc 2
−Rc
Re
∆vout
∆vin
Rc
Re R1
R2 Ve Vout
Vc Vb
Vcc= +12 V
Vin
e b c C
C
4.3 QUESTIONS
1 Briefly explain the following terms.
a linear amplification b voltage gain c clipping d current gain e inverting amplifier
2 Which of the following conditions are required for normal transistor action in an npn transistor?
a b–e junction forward biased; b–c junction forward biased
b b–e junction forward biased; b–c junction reverse biased
c b–e junction reverse biased; b–c junction forward biased
d b–e junction reverse biased; b–c junction reverse biased
3 a What is meant by the term ‘transistor amplifier biasing’?
b What is meant by the term ‘coupling capacitor’?
The following information applies to questions 4–6. The graph shows the characteristics of a voltage amplifier.
4 a Explain why the amplifier produces a constant output voltage over a range of input voltages.
b What is the maximum peak signal voltage that can be amplified without distortion?
c Calculate the voltage gain of the amplifier.
Rearranging gives:
R1=
=
=63.8 kΩ
Although the calculation is beyond the scope of this book, typical values for the two coupling capacitors used in an audio amplifier like the one shown are around 1 µF.
The voltage gain for this amplifier is approximately:
Av=
~
=
= −5
The negative sign indicates that the circuit is an inverting amplifier.
−6 kΩ 1.2 kΩ
−Rc
Re
∆vout
∆vin
12 ×103(12 −1.9) 1.9 R2(Vcc−Vb)
Vb
• For linear amplification voltage gain AV = Vout/Vin, current gain AI=Iout/Iin, voltage gain (for small time-varying signals) Av= ∆vout/∆vin.
• Single-stage transistor amplifiers are usually inverting (i.e. negative gain).
• Clipping (signal distortion) occurs with amplification that is outside the linear range.
• For a silicon npn BJT operating in the linear region:
Vbe≈0.7 V Ic≈Ie Ic>> Ib
• For a correctly biased BJT amplifier (using a voltage divider bias):
Vc~ Vcc/2 Ve~ Vcc/10 R2≤10 Re
–1.0 –0.5 0 0.5 1.0 1.5 10
5 0 –5 V (volts)out–10
Vin (volts)
4.3 SUMMARY AMPLIFICATION
5 Complete the following table by calculating the range of output voltages for the amplifier for the corresponding sinusoidal input voltages with ranges as shown.
6 The following signal is fed into the input of the amplifier.
a Draw the output signal.
b Is the output signal a faithful amplified version of the input signal?
c Describe the output waveform in relation to the input signal.
d If the amplifier is used as part of an audio system, describe the quality of the sound that it would produce.
7 The graph shows the characteristics of four different voltage amplifiers (A–D). The input voltage to these amplifiers is limited to between ±600 mV.
c Which of the four amplifiers (A–D) has linear amplification over the full range of input voltages?
d Which of the four amplifiers (A–D) has the smallest linear input range?
8 The transistor in this circuit is a silicon npn BJT with a current gain of 150.
a In the circuit shown, calculate the values of Ic, Ib and Rbthat result in a bias output voltage (Vout) of +5 V.
b If Rbis increased a little, what happens to Vout? (Choose from A–E below.)
c If Rcis decreased a little, what happens to Vout? (Choose from A–E below.)
A Voutremains unchanged.
B Voutgoes to zero.
C Voutgoes to +12 V.
D Voutincreases a little.
E Voutdecreases a little.
9 The silicon npn BJT in the circuit shown has a current gain of 200.
determine the value of Rcand the voltage gain.
10 A BJT transistor can often be damaged if the b–e junction has a large reverse bias connected across it.
Explain the function of the diode in the following transistor circuit.
Range of input voltage Range of output voltage 100 to 200 mV
200 300 400 500 600 2
1 In the following two circuits the batteries and resistors are identical. Assume the batteries are ideal (i.e. no internal resistance).
a Which resistor(s) has the highest current flowing through it?
b Which resistor(s) has the lowest current flowing through it?
c Which resistor(s) has the highest power dissipated through it?
d Which battery is supplying the largest current?
2 You have four 4 kΩresistors. How would you arrange the four resistors to give a total effective resistance of:
a 16 kΩ?
b 10 kΩ?
c 1 kΩ?
d 5.33 kΩ?
The following information applies to questions 3 and 4.
The circuit shows two resistors connected in series across a battery with a terminal voltage of V. The
resistance of RBis three times that of RA.
3 Show mathematically that of the battery’s voltage is dropped across RA.
4 If V=12 V and I=200 mA, determine:
a the resistance values of RA and RB
b the power dissipated in RB.
5 A thermistor is a semiconductor device whose resistance depends on the temperature. The graph shows the resistance versus temperature characteristic for a particular thermistor. Determine the temperature of the thermistor in the circuit if Voutis 1 V.
6 Three 100 Ωresistors are connected as shown. The maximum power that can safely be dissipated in any one resistor is 25 W.
a What is the maximum potential difference that can be applied between points A and B?
b What is the maximum power that can be dissipated in this circuit?
7 The following circuit combines variable resistors R1and R2 with fixed resistors to make a complex voltage divider. Complete the table by determining the output voltage for each row.
8 The graph represents a plot of potential difference (V) as a function of position (clockwise from X) around the electronic circuit. Assume all connecting wires and switches have zero resistance.
Complete the potential difference versus position graphs for each of the following circuits.
Temperature (°C)
20 30 40 50
a
b
9 For the circuit shown find:
a the current in the 20 Ω resistor
b the potential difference between points X and Y.
It may be useful to draw the circuit in a linear form, so that the voltage across it drops from top to bottom, as shown below.
10 Two electronic components, X and Y, are operating in the circuit shown. The voltage–current characteristics of each device are shown in the accompanying graph.
The reading on the voltmeter is 60 V.
a Which device is non-ohmic?
b What is the resistance of X?
c What is the reading on the ammeter?
d Determine the EMF of the battery.
e Calculate the total power consumption in the circuit.
11 A silicon diode has a switch-on voltage of ~0.7 V and a thermal leakage current of ~100 nA.
Determine the voltage across the diode (Vd) and the current (Id) through the diode for:
a circuit 1
b circuit 2, where the polarity of the battery has been
reversed.
12 For the circuits shown in parts a and b assume the diode has a switch-on voltage Vs=0.7 V and a thermal leakage current It=1 nA.
Determine the current flowing through the:
i 100 Ωresistor ii 500 Ωresistor iii diode.
The following information applies to questions 13 and 14. All the diodes in these circuits are identical and have the voltage–current characteristics shown in the graph.
13 In circuit A the ammeter reading is 52 mA.
a What is the reading on the voltmeter?
b What is the EMF of the battery?
14 Consider circuit B.
a Calculate the current flowing through the circuit.
b What is the reading on the ammeter?
c How much electrical energy is the battery delivering to the diodes every second?
15 Sketch the output waveform for the following circuit. Assume a Si diode with Vs=0.7 V.
16 For the circuit shown, determine the most likely semiconductor materials that the diodes (D1and D2) are made of.
17 The input–output characteristics of an amplifier are shown in the graph.
a What type of amplifier has these characteristics?
b Calculate the voltage gain for the amplifier.
c What is the maximum peak-to-peak voltage that this amplifier can amplify without distortion?
18 The graph shows the
characteristics of four different voltage amplifiers (A–D). The input voltage to these amplifiers is limited to between ±600 mV.
a If the input voltage is as shown in the diagram, sketch the output voltage for amplifiers
B and D.
b If the input voltage is as shown in the diagram, sketch the output voltage for amplifiers A and C.
20 The transistor amplifier shown has a 1 kHz sinusoidal input voltage (vin) with an amplitude of 0.5 V.
The voltage gain vout/vin= −5.
The DC bias conditions are set as follows: Ic=1 mA, Vc=6 V.
The silicon npn transistor has a current gain Ic/Ib=100. Sketch vinand voutas a function of time.
Carefully label the vertical (voltage) and horizontal (time) axes.
0.2 0.4 0.6 0.8 1.0 1.2
–100 100 300 500 –300
you will have covered material from the study of electronics and photonics including:
• conversion of electrical signals into light signals
• comparing the information-carrying capacity of copper wires and optical fibres
• using electrical energy to generate light by using semiconductor materials
• detecting light through changes in the electrical properties of semiconductor materials
• using technical specifications for photonic devices to design and build simple photonic circuits
• transmitting information (voice or music) via a light beam.
CHAPTER 5
BY THE END OF THIS CHAPTER