CHAPTER 2:

BASIC OP-AMP CIRCUITS

• Describe and analyze the operation of several types of comparator circuits.

• Describe and analyze the operation of several types of summing amplifiers.

• Describe and analyze the operation of integrators and differentiators.

Objectives:

COMPARATOR

The comparator is an op-amp circuit that compares two input voltages and produces an output indicating the relationship between them.

The inputs can be two signals (such as two sine waves) or a signal and a fixed dc reference voltage.

Comparators are most commonly used in digital applications. Digital circuits respond to rectangular or square waves, rather than sine waves. These waveforms are made up of alternating (high and low) dc levels and the transitions between them.

Comparator

The inverting (-) input is grounded to produce a zero level and the input signal voltage is applied to the noninverting (+) input as shown in Figure 1.

The incoming signal drives the amplifier into saturation producing a square-wave output.

Zero-Level Detection

Figure 1: Op-amp as a zero-level detector

When the sine wave is positive, the output is at its maximum positive level.

When the sine wave crosses 0, the amplifier is driven to its opposite state and the output goes to its maximum negative level.

Can be used as a squaring circuit to produce a square wave from a sine wave.

Figure 2: Op-amp as a zero-level detector

Connecting a fixed reference voltage source to the inverting (-) input.

Using a voltage divider to set the reference voltage, VREF:

Where +V is the positive op-amp dc supply voltage

Nonzero-Level Detection

)(21

2 VRR

RVREF

Figure 3: Nonzero-level detectors

As long as Vin is less than VREF, the output remains at the maximum negative level.

When the input voltage exceeds the reference voltage, the output goes to its maximum positive voltage.

Figure 4: Nonzero-level waveform

The input signal in Figure 5(a) is applied to the comparator in Figure 5(b). Draw the output showing its proper relationship to the input signal. Assume the maximum output levels of the comparator are ±14V.

Example 1

Figure 5

Effect of Input Noise on Comparator Operations

-Noise (unwanted voltage fluctuations appears on the input line)

- Noise can cause a comparator to erratically switch output states

Effects of noise on a zero-crossing detector

One way to reduce the effect of noise is by using a comparator with positive feedback

This circuit is usually called a Schmitt trigger

The positive feedback produces two separate trip points that prevent a noisy input from producing false transitions (i.e. UTP and LTP) – Hysteresis

UTP – upper trigger point LTP – lower trigger point

How to reduce noise effect

SCHMITT TRIGGER

(max)21

2outUTP V

RR

RV

(max)21

2outLTP V

RR

RV

+

-

R1

R2

Vin

Vout

+V

-V

Figure 6: Comparator with positive feedback for hysteresis

Determine the upper and lower trigger points for the comparator circuit in Figure 7. Assume that +Vout(max) = +5V and -Vout(max) = -5V.

Example 2

Figure 7

Answer: VUTP = +2.5V, VLTP = -2.5V

Output Bounding

The output swing of a zero-crossing detector may be too large in some applications.

In some applications, necessary to limit the output voltage levels of comparator to a value less than provided by the saturated op-amp.

We can bound the output by using a zener diode – limit the output voltage to the zener voltage in one direction

Bounded at positive value

+

_

Dz

R+V

-V

VinVout -0.7V

+Vz

0

The anode of the zener is connected to the inverting input.

When output voltage reaches positive value equal to the zener voltage – limit at that value

At negative output, zener acts as a regular diode and becomes forward biased at 0.7V – limiting the negative output voltage.

Bounded at negative value

+

_

Dz

R+V

-V

VinVout

+0.7V

-Vz

0

The cathode of the zener is connected to the inverting input.

The output voltage limits in the opposite direction.

Double-bounded

+

_

Dz1

R +V

-V

Vin

Vout

Dz2

Vz2 + 0.7V

- (Vz1 + 0.7V)

0

Two zener diodes arranged – limit the output voltage to the zener voltage plus forward biased 0.7V (positively and negatively).

Determine the output voltage waveform for Figure 8.

Example 3

Figure 8

Comparator Applications

Over Temperature Sensing Circuit

Analog-to-Digital (A/D) Conversion

SUMMING AMPLIFIERS

Summing amplifier has two or more inputs.

Its output voltage is proportional to the negative sum of its input voltages.

VOUT = - (VIN1 + VIN2 + VIN3 + … + VINn)

Figure 9: Summing amplifier with n inputs

When Rf is larger than the input resistors, the amplifier has a gain of Rf/R.

Summing amplifier with gain greater than unity

Nfout VVV

R

RV .....21

1

Averaging Amplifier A summing amplifier can be made to

produce the average of the input voltages. n = number of inputs

Rf/R = 1/n

Is a summing adder with each input having different gain

The Rf to input resistance ratio would determine what the voltage output would be with a signal present at each output.

Scaling adder

INn

n

fIN

fIN

fOUT V

R

RV

R

RV

R

RV ...2

21

1

Determine the output voltage for the summing amplifier in Figure 10 (a) and (b).

Example 4

Figure 10 (a) Figure 10 (b)

OP-AMP INTEGRATOR

The feedback element is a capacitor that forms an RC circuit with the input resistor.

Ideal Integrator

Ideal Integrator

•When a constant positive step input voltage is applied, the output ramp decreases negatively until the op-amp saturates at its maximum negative level.

•The integrator can be used to change a square wave input into a triangular wave output.

•The rate of change of the output voltage:

CR

V

t

V

i

inout

(a) Determine the rate of change of the output voltage in response to the input square wave, as shown for ideal integrator in Figure above. The output voltage is initially zero. The pulse width is 100us.

(b) Describe the output and draw the waveform.

Example 5

Use a resistor in parallel with the capacitor in the feedback path.

The feedback resistor Rf, should be large compared to the input resistor, Rin, in order to have a negligible effect on the output waveform.

Practical Integrator

–

+

Vin

R

R

Vout

C

f

OP-AMP DIFFERENTIATOR

The capacitor is the input element, and the resistor is the feedback element.

A differentiator produces an output that is proportional to the rate of change of the input voltage.

Ideal Differentiator

CRt

VV f

Cout

•When input is a positive-going ramp, the output is negative (capacitor is charging)

•When input is a negative-going ramp, the output is positive (capacitor is discharging) – current is the opposite direction

Determine the output voltage of the ideal op-amp differentiator in Figure above for the triangular-wave input shown.

Example 6

Adding Rin, in series with the capacitor to act as a low-pass filter and reduce the gain at high frequencies.

The resistor should be small compared to the feedback resistor in order to have a negligible effect on the desired signal.

Practical Differentiator

–

+

C

Vout

Vin

R

R

R

in

c

f

~ End of Chapter 2 ~