Transcript 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.

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.
Figure 1: Op-amp as a zero-level detector


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.
Figure 2: 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 3: Nonzero-level detectors


Connecting a fixed reference voltage source
to the inverting (-) input.
Using a voltage divider to set the reference
voltage, VREF: VREF  R2 (V )
R1  R2

Where +V is the positive op-amp dc supply
voltage
Figure 4: Nonzero-level waveform


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 5
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.
-Noise (unwanted
voltage fluctuations
appears on the input
line)
- Noise can cause a
comparator to erratically
switch output states
How to reduce noise
effect






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
+V
Vin
-
Vout
+
-V
R1
R2
VUTP
R2
 Vout(max) 

R1  R2
VLTP 
R2
 Vout(max) 
R1  R2
Figure 6: Comparator with positive feedback for hysteresis
Figure 7
Determine the upper and lower trigger
points for the comparator circuit in Figure 7.
Assume that +Vout(max) = +5V and -Vout(max) =
-5V.
Answer: VUTP = +2.5V, VLTP = -2.5V



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
Dz
Vin
R
+Vz
+V
_
0
Vout
-0.7V
+
-V



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.
Dz
Vin
R
_
+
V
Vou
+

0
t
-V

+0.7V
-Vz
The cathode of the zener is connected to
the inverting input.
The output voltage limits in the opposite
direction.
Dz
1
R
Vin
_
Dz2
+
V
Vz2 +
0.7V
Vou
+
t
-V

0
- (Vz1 +
0.7V)
Two zener diodes arranged – limit the
output voltage to the zener voltage plus
forward biased 0.7V (positively and
negatively).
Figure 8
Determine the output voltage waveform for
Figure 8.
*Hint: Vout = VR1 + VR2
Analog-to-Digital (A/D) Conversion
Over Temperature Sensing Circuit
Figure 9: Summing amplifier with n inputs
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Summing amplifier has two or more inputs.
Its output voltage is proportional to the
negative sum of its input voltages.
If Rf = R1 = R2 = R3 = Rn, Vout can be expressed
as,
VOUT = - (VIN1 + VIN2 + VIN3 + … + VINn)

When Rf is larger than the input resistors,
the amplifier has a gain of Rf/R.
Vout  
Rf
R
V1  V2  .....  VN 
where R1 = R2 = R3 = Rn = R


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.
VOUT
Rf
Rf
 Rf


  VIN 1 
VIN 2  ... 
VINn 
R2
Rn
 R1

Figure 10 (a)
Figure 10 (b)
Determine the output voltage for the
summing amplifier in Figure 10 (a), (b) and
(c).
Figure 10 (c)
Figure 10 (c)

The feedback element is a capacitor that
forms an RC circuit with the input resistor.
•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:
Vout
Vin

t
Ri C
(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.
Rf
C
R
Vin
–
Vout
+


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.


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.
Vout
 VC 
   R f C
 t 
•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.
Rf
Vin
Rin
C
–
Vout
+
Rc


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.