OPERATIONAL AMPLIFIERS - OP-AMP PERFORMANCE

TABLE A lists parameters for two different operational amplifiers, A and B.

Question 1. a) Contrast the relative performance of each op-amp in terms of these parameters and state how they compare to that of the ideal op-amp.

b) TABLE B gives two descriptions of operational amplifiers. State, with justifications, which description most closely matches the op-amps of TABLE A.

c) State, with reasons, which op-amp in TABLE A would be most suitable for the applications listed in TABLE C.

d) TABLE D shows the internal circuitry of two op-amps. Identify, with justification, which op-amp circuit is most likely to belong to which op-amp in TABLE A.

Question 2. The amplifier of FIGURE 1 has a closed loop gain of 20 dB and R1 = 10 kΩ. The resistors have a tolerance of 5%.

a. Estimate the output offset voltage of the circuit of FIGURE 1 when the op-amp used is
i) op-amp A of TABLE A
ii) op-amp B of TABLE A.

b. If the circuit of FIGURE 1 uses op-amp A of TABLE A, show how the output offset voltage can be reduced by a circuit modification, calculating the value of any components used.

c. Explain why the technique used in (b) above is relatively ineffectual if op-amp B of TABLE A is used.

N.B. The output offset voltage is given by the equation:

V_2O = I_IB(R_f - R₂((R_f + R₁)/R₁) + I_IOR_f + V_IO(1 + R_f/R₁))

Figure 1

Question 3. a) Explain what is meant by ‘unconditional stability' in an op-amp.

b) FIGURE 2 gives the open-loop response of an uncompensated op- amp. Compensation is to be applied to the amplifier to make it unconditionally stable. Estimate the frequency at which the breakpoint of the compensated response must occur.

FIGURE 2

Question 4. FIGURE 3(a) shows the Bode plot of op-amp THS4021. The manufacturer describes this device as a ‘decompensated op-amp', by which is meant that the frequency compensation is partial as opposed to ‘uncompensated' where no compensation is used at all. The advantages of ‘decompensation' over ‘full compensation' include a higher open-loop gain and better slew rate. Greater care, however, is need in the use of such an op-amp as frequency stability cannot be assured.

a) Estimate the phase margin and unity gain BW of the THS 4021.

b) FIGURE 3(b) shows the THS4021 in a practical op-amp circuit with applied external compensation. Determine an expression for the low frequency gain of the circuit.

Figure 3(a)

Figure 3(b)

Question 5. In FIGURE 4(a) the input voltage V_in is swept from 0 to 5 V. The supply voltage is a single-rail 5 V. Explain the shapes of the output responses of the two op-amps shown in FIGURE 4(b).

[TABLE E gives some further information on these op-amps.]

Question 6. Estimate the maximum frequency of a sinusoidal input signal of 10 mV peak, for distortion-free output, when applied to an op-amp circuit of closed-loop gain of 100, if the op-amp used is the:

a) TL072
b) TLV2362.

[TABLE E gives some further information on these op-amps.]

Figure 4(a)

Figure 4(b)

Attachment:- OP-AMP PERFORMANCE.rar