_____________________________________________________________________ Special Amplifiers
Compute the voltage developed by R3 (ER3):
ER3 = (R3) x (IR3)
ER3 = (I kΩ) x (+3 mA)
ER3 = +3 V
Add this to the voltage at point A to compute the output voltage (EOUT):
EOUT = (ER3) + (voltage at point A)
EOUT = (+3 V) + (+6 V)
EOUT = +9 V
7-94. The circuit shown in Figure 7-24 functions as a subtractor. Just as an adder is only
one kind of summing amplifier, a subtractor is only one kind of difference amplifier. A
difference amplifier can amplify the difference between two signals. For example, with two
inputs (E1 and E2) and a gain of five, a difference amplifier will produce an output signal
that is:
EOUT = 5 (E2 - E1)
7-95. The difference amplifier that will produce that output is shown in Figure 7-25.
Notice that this circuit is the same as the subtractor shown in Figure 7-24 except for the
values of R3 and R4. The gain of this difference amplifier is:
R3
Gain =
R1
5k Ω
Gain =
1k Ω
Gain = 5
and since:
R3
R4
=
R1
R2
then, for a difference amplifier:
R3
R4
Gain =
=
R1
R2
23 June 2005
TC 9-62
7-33
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