FT009/2
Student Name:
Industrial Electronics Laboratory
Lab No. 7
Objective:
Equipment
required:
Group: _____
Comparator
To observe the performance of an op-amp comparator circuit.
+/- 12 volt power supply unit
Oscilloscope
DVM
Function Generator
Test Leads
Background:
A comparator is a circuit that performs a comparison between two voltages (V1 and V2) and
provides an output that indicates which of the two voltages is greater by switching its output
either high (if V1 > V2) or low (if V1 < V2).
An operational amplifier may be used to perform this function.
+V
+
-
V1
V2
Vout
-V
Figure 1
The op amp gives:
Vout = AOL .(V1 − V2 )
If V1>V2, Vout will swing up to the positive output saturation voltage +VSAT (+VSAT ≈ +V).
If V2>V1, Vout will swing down to the negative output saturation voltage -VSAT (-VSAT ≈ -V).
Thus for any values of V1 and V2 (except V1 = V2), Vout will go to either +VSAT (high) or –VSAT
(low). High or low values for Vout may be used as a signal to show that either V1>V2 or V1<V2
respectively.
If V1 = V2, the output is uncertain. This may be overcome by using positive feedback to add
some hysteresis to the transfer characteristic –
FT009/2
Student Name:
Industrial Electronics Laboratory
Group: _____
R2
+V
R1
+
-
V1
V2
Vout
-V
Figure 2
This circuit will switch the output low if V1 falls below a threshold voltage:
R
R + R2
VTHL = 1
.V2 + 1 .VSAT
R2
R2
and the circuit will switch the output high if V1 rises above a threshold voltage:
R + R2
R
VTHU = 1
.V2 − 1 .VSAT
R2
R2
(Note that if R2 is open circuit (R2 = ∞) VTHU = VTHL = V2.)
Procedure:
1. Build the circuit in figure 1 and with V2 = 5 volts, +V = 12 volts, -V = -12 volts.
2. Plot VOUT as a function of V1 and comment on your results.
3. Build the circuit in figure 2 with R1 = 1k, R2 = 10k,V2 = 5 volts, +V = 12 volts, -V = 12 volts.
4. Plot VOUT as a function of V1 and comment on your results.
5. Connect V1 = 10 volt peak sine wave at 1 kHz with other voltages as before.
6. Measure both V1 and VOUT on an oscilloscope and sketch both waveforms to scale.
Comment on the relationship between the two signals.
FT009/2
Student Name:
Industrial Electronics Laboratory
Figure 1 Characteristic
Comment
Figure 2 Characteristic
Comment
Group: _____
FT009/2
Student Name:
Industrial Electronics Laboratory
Figure 2 Input and Output
Comment
Group: _____