A-LevelPhysicsD.C. circuits9702/41 · Oct/Nov 2017 · Q711 Marks

The circuit of an amplifier incorporating an ideal operational amplifier (op-amp) is shown in Fig. 7.1. [Figure 7.1]

(a)(i)By reference to the properties of an ideal op-amp, explain why point P is referred to as a virtual earth,[4]
(a)(ii)derive an expression, in terms of the resistances R₁ and R₂, for the gain of the amplifier circuit.[4]
(b)In the circuit of Fig. 7.1, the ratio R₂/R₁ is 4.5. The variation with time t of the input potential V_IN is shown in Fig. 7.2. [Figure 7.2] On Fig. 7.2, show the variation with time t of the output potential V_OUT.[3]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2017). The mark or mark type is shown in brackets.

[B1] gain of amplifier is very large
[B1] V⁻ is at earth (potential)
[M1] for amplifier not to saturate
[A1] difference between V⁻ and V⁺ must be very small or V⁻ must be equal to V⁺
[B1] input impedance is infinite
[B1] (so) current in R₁ = current in R₂
[B1] (V_IN - 0) / R₁ = (0 – V_OUT) / R₂
[B1] (gain =) V_OUT/ V_IN = – R₂/R₁
[B1] graph: correct inverted shape (straight diagonal line from (0,0) to a negative potential, then a horizontal line, then a straight diagonal line back to the t-axis at the point where V_IN = 0)
[B1] horizontal line at correct potential of (-)9.0V
[B1] both ends of horizontal line occur at correct times (coinciding with when V_IN = 2.0V)

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