Engineering and

Architecture

Analog

Electronics

Circuits

EEE321

Lec 9

Operational Amplifier II

Lecturer Sally Adil

– The output of a linear op-amp circuit has the same shape

as the input signal. At no time during the cycle does the op

amp go into saturation.

– The output of a nonlinear op-amp circuit usually has a

different shape from the input signal because the op-amp

saturates during part of the input cycle.

Summing Amplifier

❖The summing amplifier is an application of the inverting op-

amp configuration

❖A summing amplifier has two or more inputs, and its output

voltage is proportional to the negative of the algebraic sum

of its input voltages.

1.Summing Amplifier with

Unity Gain

– Two voltages, Vin1 and Vin2 are applied to the inputs and produce

currents I1 and I2, as shown. Using the concepts of infinite input

impedance and virtual ground, you can determine that the inverting

input of the op-amp is approximately 0 V and has no current through it.

This means that the total current IT, which goes through Rf divides into

I1 and I2 at summing point A,

1.Summing Amplifier with

Unity Gain

– for a unity-gain summing amplifier with n inputs where all

resistors are equal in value.

2. Summing Amplifier with Gain

Greater Than Unity

❖When Rf is larger than the input resistors, the amplifier has

a gain of Rf /R, where R is the value of each equal-value

input resistor. The general expression for the output is

Summing Amplifier with Gain

Greater Than Unity

3. Averaging Amplifier

– An averaging amplifier is basically a summing amplifier

with the gain set to Rf /R = 1/n (n is the number of inputs).

The output is the negative average of the inputs.

4 .Scaling Adder

❖A scaling adder has two or more inputs with each input

having a different gain. The output represents the negative

scaled sum of the inputs.

❖The weight of a particular input is set by the ratio of Rf to

the resistance, Rx.

Applications of Summing

amplifier

❖ D/A conversion is an important interface process for

converting digital signals to analog (linear) signals

❖ Method of D/A conversion

1. binary-weighted resistor DAC).

2. the R/2R ladder method

Comparator

– A comparator is a specialized nonlinear op-amp circuit that

compares two input voltages and produces an output state

that indicates which one is greater. Comparators are

designed to be fast and frequently have other capabilities to

optimize the comparison function.

Comparator

❖ Because the output is always in one of two states,

comparators are often used to interface between an analog

and digital circuit

❖ For less critical applications, an op-amp running without

negative feedback (open-loop)is often used as a comparator.

❖ In general, comparators cannot be used as op-amps, but op-

amps can be used as comparators in noncritical applications

Why? Because an op-amp without negative feedback is

essentially a comparator,

Zero-Level Detection

– the inverting input is grounded to produce a zero level and that the

input signal voltage is applied to the noninverting input.

– Because of the high open-loop voltage gain, a very small difference

voltage between the two inputs drives the amplifier into saturation,

causing the output voltage to go to its limit.

The op-amp as a zero-level detector.

Nonzero-Level Detection

Comparator with Hysteresis

– Sometimes the input signal to a comparator may vary due to noise superimposed on the input. The result can be an unstable output. To avoid this, hysteresis can be used

– Hysteresis is incorporated by adding regenerative (positive) feedback,

which creates two switching points:

the upper trigger point (UTP)

and the lower trigger point (LTP).

After one trigger point is crossed,

it becomes inactive and the other

one becomes active.

Comparator with Hysteresis

– A comparator with hysteresis is also called a Schmitt

trigger. The trigger points are found by applying the

voltage-divider rule:

( )2UTP ( )

1 2

out max

RV V

R R= +

+

( )2LTP ( )

1 2

out max

RV V

R R= −

+

Output Bounding

– Some applications require a limit to the output of the

comparator (such as a digital circuit). The output can be

limited by using one or two zener diodes in the feedback

circuit.

– The circuit shown here is bounded as a positive value equal

to the zener breakdown voltage.

Comparator Applications

– Simultaneous or flash analog-to-digital

Integrator

– The ideal integrator is an inverting amplifier that has a

capacitor in the feedback path. The output voltage is

proportional to the negative integral (running sum) of the

input voltage.

The output is the integral of the

input; i.e., proportional to the

area under the input waveform.

This circuit is useful in low-

pass filter circuits and sensor

conditioning circuits.

−= dttvRC

tvo )(1

)( 1

Integrator

– Op-amp integrating circuits must have extremely low dc

offset and bias currents, because small errors are equivalent

to a dc input. The ideal integrator tends to accumulate these

errors, which moves the output toward saturation. The

practical integrator overcomes these errors– the simplest

method is to add a relatively large feedback resistor.

Integrator

– If a constant level is the input, the current is

constant. The capacitor charges from a constant

current and produces a ramp. The slope of the

output is given by the equation:

out in

i

V V

t R C

= −

Differentiator

– The ideal differentiator is an inverting amplifier that has a

capacitor in the input path. The output voltage is

proportional to the negative rate of change of the input

voltage.

The differentiator takes

the derivative of the input.

This circuit is useful in

high-pass filter circuits.

dt

tdvRCtvo

)()( 1−=

Differentiator

– The small reactance of C at high frequencies means an ideal

differentiator circuit has very high gain for high-frequency

noise. To compensate for this, a small series resistor is often

added to the input. This practical differentiator has

reduced high frequency gain and is less prone to noise.

Differentiator

– The output voltage of a differentiator is given by

Cout f

VV R C

t

= −

Thank you