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S7 H.W. 2
1.
http://www.hkedcity.net/ihouse/kc6677
The circuit shows a linear voltage amplifier formed by a transistor of current
gain  = 120. When no signal is applied to the transistor, the output voltage is set
at 3 V. Calculate
a. the voltage gain Vout/Vin of the amplifier.
b. the maximum peak voltage of the input signal.
c. the biased current at the input, given that VBE = 0.7 V.
d. the input voltage Vin with no signal applied.
State the functions of the capacitors C1 and C2.
2.
[-15; 0.2 V; 10 A; 0.9 V]
The diagram shows an op amp with the connections to the dual voltage power
supply omitted. All the voltages are relative to the earth wire of the power supply.
The maximum output voltage of the op amp is 15 V.
a.
b.
If V1 = 3 V and V2 = 1 V, what is the voltage at the output?
The V-t graphs show how V1 and V2 vary with time. Sketch a V-t graph to
show how the output voltage varies with time.
[-15 V]
3.
4.
An inverting amplifier is connected as shown.
a.
b.
What are the currents through the resistors?
Find the voltage Vout at the output.
c.
A buzzer of resistance 1 k is connected across the output of the amplifier.
i.
Indicate the current direction through the buzzer.
ii.
What is the power consumed by the buzzer?
[75 A, 75 A; -7.5 V upward; 56.25 mW]
An LED would emit light when the current through it is at least 4 mA. It is
inserted into the feedback loop of a non-inverting amplifier as shown. The
forward-bias p.d. of the LED is negligible.
a.
b.
What is the required input voltage Vin to light up the LED?
What is the voltage at the output if the LED just lights up?
c.
Given that the maximum output voltage of the op amp is 15 V, what is the
largest possible current through the LED?
d.
Find the current in RL when
i.
the LED is just light up.
ii.
the maximum current flows through the LED.
[-4 V; -12 V; 5 mA; 12 mA, 15 mA]
Please hand in by
20 October, 2003