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Amplifiers
A.3.18 Inverting AC Amplifier
V CC and resistors R set a dc level of V CC /2 at the inverting input. R G is connected to
ground through a capacitor, thus the circuit functions as a buffer for dc. This causes the
dc output voltage to be V CC /2, so the quiescent output voltage is the middle of the supply
voltage, and it is ready to swing to either rail as the input signal commands.
The ac gain is given in Equation A–25. R G and C form a coupling network for the ac signal.
Good coupling networks should be constant low impedance at the signal frequencies, so
Equation A–26 should be satisfied to get good low-frequency performance. The lowest
frequency component of the input signal, f MIN , is determined by completing a Fourier series
on the input signal. Then, setting f MIN = 100f in Equation A–26 ensures that the 3-dB
breakpoint introduced by R G and C is two decades lower than f MAX .
R
V OUT
V F
IN
V CC
R G
2
f
1
200R G
C
A–25
A–26
C
R G
V IN
+V CC
R
R
R F
+V CC
_
+
V OUT
Output Amplitude — dB
0
–2
–4
–6
–8
–10
–12
–14
–16
–18
V OUT
Phase
–20 170
10 100 1 k 10 k 100 k
f – Frequency – Hz
V IN = 1 Vp-p
C = 0.01 F
V CC = 5 V R G = R F = R = 10 k
Op Amp = TLV247x
270
260
250
240
230
220
210
200
190
180
Phase — Degrees
Figure A–22. Inverting AC Amplifier
A-22