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Ideal Operational

Amplifiers and

Op-Amp Circuits

Ch.9 , Page - 621

Department of Electrical Engineering

Historical Review of Op Amp !!

The μA-709 was introduced by Fairchild Semiconductor in 1965 and was one of the first widely used general-purpose op-amps

Now a days there is a wide series of Op Amps depending upon the area of use

The op-amp also requires dc power, as do all transistor circuits, so that the transistors are biased in the active region. Also, most op-amps are biased with both a positive and a negative voltage supply

741 Op Amp IC Chip

Ideal Parameters

Ideally, the input resistance Ri between terminals 1 and 2 is infinite, which means that the input current at each terminal is zero.

The output terminal of the ideal op-amp acts as the output of an ideal voltage source, meaning that the small-signal output resistance Ro is zero

 Example

Common-mode Input Signal

 Since the ideal op-amp responds only to the difference

between the two input signals v1 and v2, the ideal op- amp maintains a zero output signal for v1 = v2.

 When v1 = v2 ≠ 0, there is what is called a common-

mode input signal. For the ideal op amp, the

common-mode output signal is zero. This characteristic

is referred to as common-mode rejection

Development of the Ideal

Parameters

The voltage at the non-inverting terminal is set equal to zero, so that the non inverting terminal is at ground potential. An input voltage vI is applied

Conclusions drawn from

 If we let the gain gm  ∞ , thenVgs ≈ 0; that is, the voltage

at the input terminal to the basic amplifier is almost at

ground potential.

 This terminal is said to be at virtual ground

Equivalent circuit determining output

resistance

Conclusion drawn from

 If the gain gm  ∞ , then Ro → 0. The output resistance of

the circuit with negative feedback included goes to zero. This

is also a property of an ideal op-amp circuit.

Analysis Method

 Usually, an op-amp is not used in the open-loop configuration.

 Instead, feedback is added to close the loop between the output

and the input.

Negative feedback, in which the connection from the output goes to the inverting terminal, or terminal (1). This configuration produces stable circuits. Positive feedback , in which the output is connected to the non-inverting terminal, can be used to produce oscillators.