
HA17902 Series
12
3. High Input Impedance DC Differential Amplifier
The circuit shown in figure 3 is a high input impedance DC differential amplifier. This circuit’s
common-mode rejection ratio (CMR) depends on the matching between the R
1
/R
2
and R
4
/R
3
resistance
ratios. This amplifier’s output is given by the following formula.
R
4
R
3
Vout = 1 +
(V
2
– V
1
)
+
–
+
–
R
2
100k
100k
R
4
R
1
100k
R
3
100k
Vout
V
1
V
2
Figure 3 High Input Impedance DC Differential Amplifier
4. Voltage Controlled Oscillator
Figure 4 shows an oscillator circuit in which the amplifier A
1
is an integrator, the amplifier A
2
is a
comparator, and transistor Q
1
operates as a switch that controls the oscillator frequency. If the output
Vout1 is at the low level, this will cut off transistor Q1 and cause the A
1
inverting input to go to a higher
potential than the noninverting input. Therefore, A
1
will integrate this negative input state and its output
level will decrease. When the A
1
integrator output becomes lower than the A
2
comparator noninverting
input level (V
CC
/2) the comparator output goes high. This turns on transistor Q
1
causing the integrator to
integrate a positive input state and for its output to increase. This operation generates a square wave on
Vout1 and a triangular wave on Vout2.
+
–
–
+
100k
10k
51k
51k
R
100k
V
CC
A
2
V
CC
A
1
Vout1
Vout2
V
CC
/2
HA17902
HA17902
R/2
50k
Q
1
+V
C
C 0.05
μ
F
Figure 4 Voltage Controlled Oscillator