Fig. 8.1 shows a circuit incorporating an ideal operational amplifier (op-amp).

Question 5

(a) Fig. 8.1 shows a circuit incorporating an ideal operational amplifier (op-amp).

Fig. 8.1

The voltages applied to the inverting and the non-inverting inputs are V1 and V2 respectively.

State the value of the output voltage VOUT when

(i) V1 > V2,

(ii) V1 < V2. [1]

(b) The circuit of Fig. 8.2 is used to monitor the input voltage VIN.

Fig. 8.2

At point A, a potential of 5.0 V is maintained. At point B, a potential of 3.0 V is

maintained.

Complete Fig. 8.3 by indicating with a tick (✓) the light-emitting diodes (LEDs) that are

conducting for the input voltages VIN shown. Also, mark with a cross (x) those LEDs that

are not conducting.

VIN / V              red LED           green LED

+2.0

+4.0

+6.0

[3]

Fig. 8.3

(c) The input voltage VIN in (b) is provided by a sensor circuit.

(i) Complete Fig. 8.4 to show a sensor circuit that will provide a voltage output that

increases as the temperature of the sensor decreases. Show clearly the output

connections from the circuit. [2]

Fig. 8.4

(ii) Explain the operation of the sensor circuit. [3]

Reference: Past Exam Paper – November 2007 Paper 4 Q8

Solution:

(a)

(i) - 9 V
{This is a comparator circuit as voltages are applied to both the inverting and non-inverting voltages.

When V₂ is greater than V₁, the output is positive and when V₁ is greater than V₂, the output is negative.
Since the one of the voltages is greater than the other, the output would be saturated and thus, equal to the supply voltage of 9V.}

(ii) + 9 V

(b)

V_IN / V             red LED          green LED

+2.0                 cross                cross               

+4.0                 tick                  cross               

+6.0                 tick                  tick                 

{Current flows from a positive potential to a negative potential. Since the bottom line is at earth (0 V), for current to flow from the op-amp to the earth line, the output voltage from the op-amp should be positive.

So, both LEDs would conduct when the output is positive.

V_IN is connected to the non-inverting input (V⁺) of both op-amps.

The inverting input V- for the op-amp connected to the red LED is at 3.0 V while that for the op-amp connected to the green LED is at 5.0 V.

So, for the red LED to light up, V_IN should be greater than 3.0 V (so that the output is positive) and for the green LED to light up. V_IN should be greater than 5.0 V (so that the output is positive).}

(c)

(i)

A thermistor and a resistor is shown in series.

The output connections are across the thermistor.

(ii)

As the temperature decreases, the resistance of the thermistor increases.

The potential difference across the thermistor = V × [R_T / (R+R_T)]. 

So, as the resistance of the thermistor R_T increases, the output also increases.