Operational Amplifiers

Transcription

1 Fundamentals of op-amp Operation modes Golden rules of op-amp Op-amp circuits Inverting & non-inverting amplifier Unity follower, integrator & differentiator

2 Introduction An operational amplifier, or op-amp, is a very high gain differential amplifier with high input impedance and low output impedance. Shown in the figure is a basic op-amp with two inputs and one output. Each input results an output either in phase or out of phase with respect to the input.

3 Figure: Basic op-amp

4 Single-ended input Operation Modes Only one input is applied with input signal while the other is connected to ground. The input applied to the plus input results in an output having the same polarity as input. The input applied to the minus input results in an output being opposite in phase to the applied signal.

5 Figure: Single-ended input

6 Double-ended input When signal is applied between both inputs, it is referred to as double-ended input. The amplified output is in phase with the difference between the two inputs. This is the reason that this mode is called differential input.

7 Figure: Double-ended input

8 Double-ended output The op-amp can also be operated with opposite outputs. The figure shows a single-ended input with a double-ended output. The signal applied to the plus input results in two amplified output of opposite polarity.

9 Figure: Double-ended output with single-ended input

10 Common-mode When the same input signals are applied to both inputs, common-mode operation results. Ideally, the output is zero due to the two opposite output components. This means that signals common to both inputs will be suppressed, referred to as common-mode rejection.

11 Figure: Common-mode operation

12 Basic Op-amp Connection The basic circuit connection using an opamp is shown in the figure. An input signal v 1 is applied through resistor R 1 to the inverting input terminal. The output is connected back to the inverting input terminal through resistor R f. The noninverting input terminal is connected to the ground.

13 With the assumption that: Input impedance is infinite. The output impedance is zero. The voltage gain is infinite. We get v v o R f 1 R 1 which means that the relationship is only determined by the two resistors.

14 Golden rules If there is negative feedback and if the output is not saturated, it is true that: both inputs are at the same voltage; no current flows in or out of either input. This leads to the virtual short-circuit between the two inputs. And the inverting terminal is at virtual ground.

15 Figure: Basic op-amp connection

16 Inverting Amplifier Common Connections As discussed previously, the inverting amplifier is shown in the figure. The output is obtained by multiplying the input by a fixed gain, set by R 1 and R f. v o R R f 1 v 1

17 Figure: Inverting amplifier

18 Non-inverting Amplifier The non-inverting amplifier is shown in the figure. The input signal is fed through noninverting input terminal. With the concepts of virtual short-circuit, the output is obtained R f vo ( 1 ) v1 R 1

19 Figure: Non-inverting amplifier

20 Unity Follower The unity-follower circuit is shown in the figure. It provides a gain of unity with no polarity or phase reversal. With the concepts of virtual short-circuit, the output is obvious. v o v 1

21 Figure: Unity follower

22 Voltage Summing 11. Op-amp Applications Based on the inverting amplifier, the voltage summing is shown in the figure. The output is the sum of the three inputs, each multiplied by a independent gain. v o R f R f R f ( v ) 1 v2 v3 R R R 1 2 3

23 Figure: Summing amplifier

24 Integrator Based on the inverting amplifier and with the feedback resistor replaced by a capacitor, it is integrator as shown in the figure. For a capacitor, we know that dvc ( t) ic ( t) C dt Also from the circuit, we get v1 i C ( t) R

25 And that v o = -v C (t). So, we obtain that That is Then, v dv t C ) 1 ( dv C C o R dt dt dv o v 1 dt RC 1 v o ( t) v1( t) dt RC

26 Figure: Integrator

27 Differentiator After the investigation of integrator, exchanging the position of the resistor and capacitor, it is differentiator circuit as shown in the figure. We obtain that dv1( t) v o ( t) RC dt

28 Figure: Differentiator

29 Summary Operation modes Single-ended input, double-ended input Common-mode Golden rules of op-amp Op-amp circuits Inverting & non-inverting amplifier Unity follower, integrator & differentiator