HA17902 Series
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 R1/R2 and R4/R3 resistance ratios. This amplifier’s
output is given by the following formula.
Vout = 1 + R4
R3
(V2 – V1)
R2
R4
R1
100kΩ
100kΩ
–
100kΩ
R3
–
V1
+
100kΩ
+
V2
Vout
Figure 3 High Input Impedance DC Differential Amplifier
4. Voltage Controlled Oscillator
Figure 4 shows an oscillator circuit in which the amplifier A1 is an integrator, the amplifier A2 is a comparator, and
transistor Q1 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 A1 inverting input to go to a higher potential than the noninverting input.
Therefore, A1 will integrate this negative input state and its output level will decrease. When the A1 integrator
output becomes lower than the A2 comparator noninverting input level (VCC/2) the comparator output goes high.
This turns on transistor Q1 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.
+VC
R
100k
51k
R/2
50k
C 0.05µF
100k
VCC
– A1
VCC
HA17902
– A2
+
VCC/2
HA17902
+
51k
Q1
10k
Vout1
Vout2
Figure 4 Voltage Controlled Oscillator
Rev.1.00 Jun 15, 2005 page 9 of 10