Voltage Levels and Saturating Property of Op-amp

Voltage Levels and Saturating Property of Op-amp

The supply voltages of the op-amp are + V_CC and - V_EE. These supply voltage levels decide the maximum output voltage levels of the op-amp. 

Key Point Practically the op-amp output saturates at the voltages slightly less than the supply voltages + V_CC and -V_EE.

Thus the output voltage of the op-amp can be driven to within 1 V of + V_CC and - V_EE before the output saturation takes place. This is called saturating property of the op-amp.

Practically maximum output swing of the op-amp is 0.9 times the supply voltages. Thus the op-amp using  ± 15 V supply has maximum output voltage swing as 0.9 × (± 15 V) i.e. ± 13.5 V.

Ideally it is considered that the output voltage of the op-amp saturates at + V_CC rnd - V_EE levels.

The output voltage levels are shown in the Fig. 2.9.1.  

Ideal Op-amp Characteristics

The Fig. 2.10.1 shows an ideal op-amp. It has two input signals V₁ and V₂ applied to non-inverting and inverting terminals, respectively.

The following things can be observed for the ideal op-amp shown in the Fig. 2.10.1.

1) An ideal op-amp draws no current at both the input terminals i.e. I₁ = I₂ = 0. Thus its input impedance is infinite. Any source can drive it and there is no loading on the driver stage.

2) The gain of an ideal op-amp is infinite (∞), hence the differential input V_d=V₁-V₂ is essentially zero for the finite output voltage V_o

3) The output voltage V_o is independent of the current drawn from the output terminals. Thus its output impedance is zero and hence output can drive an infinite number of other circuits.

These properties are expressed generally as the characteristics of an ideal op-amp. The various characteristics of an ideal op-amp are :

a) Infinite voltage gain : (A_OL = ∞)

It is denoted as A_OL. It Is the differential open loop gain and is infinite for an ideal op-amp.

b) Infinite input impedance : (R_in = ∞)

The input impedance is denoted as Rin and is infinite for an ideal op-amp. This ensures that no current can flow into an ideal op-amp.

c) Zero output impedance : (R_o = 0)

The output impedance is denoted as R_o and is zero for an ideal op-amp. This ensures that the output voltage of the op-amp remains same, irrespective of the value of the load resistance connected.

d) Zero offset voltage : (V_ios = 0)

The presence of the small output voltage though V₁ = V₂ = 0 is called an offset voltage. It is zero for an ideal op-amp. This ensures zero output for zero input signal voltage.

e) Infinite bandwidth :

The range of frequency over which the amplifier performance is satisfactory is called its bandwidth. The bandwidth of an ideal op-amp is infinite. This means the operating frequency range is from 0 to This ensures that the gain of the op-amp will be constant over the frequency range from d.c. (zero frequency) to infinite frequency. So op-amp can amplify d.c. as well as a.c. signals.

f) Infinite CMRR : (p = ∞)

The ratio of differential gain and common mode gain is defined as CMRR. Thus infinite CMRR of an ideal op-amp ensures zero common mode gain. Due to this common mode noise output voltage is zero for an ideal op-amp.

g) Infinite slew rate : (S = ∞)

This ensures that the changes in the output voltage occur simultaneously with the changes in the input voltage.

The slew rate is important parameter of op-amp. When the input voltage applied is step type which changes instantaneously then the output also must change rapidly as input changes. If output does not change with the same rate as input then there occurs distortion in the output. Such a distortion is not desirable. Infinite slew rate indicates that output changes simultaneously with the changes in the input voltage.

The parameter slew rate is actually defined as the maximum rate of change of output voltage with time and expressed in V/µs.

Its ideal value is infinite for the op-amp.

h) No effect of temperature :

The characteristics of op-amp do not change with temperature.

i) Power Supply Rejection Ratio : (PSRR = 0)

The power supply rejection ratio is defined as the ratio of the change in input offset voltage due to the change in supply voltage producing it, keeping other power supply voltage constant. It is also called power supply sensitivity.

So if V_EE is constant and due to change in V_CC, there is change in input offset voltage then PSRR is expressed as,

For a fixed V_CC, if there is change in V_EE causing change in input offset voltage then,

It is expressed in mV/V or µV/V and its ideal value is zero.

These ideal characteristics of op-amp are summarized in the Table 2.10.1.

Review Question

1. What are ideal characteristics of op-amp ?