Service manual

STP 11-25R13-SM-TG
Q - 37
c. Characteristics.
(1) Open loop voltage gain (A
VOL
) is the differential gain of the op amp with no feedback. The
formula for this is:
A
VOL
= V
O
V
id
V
id
= V
l
- V
2
(2) Note Figure Q-33 shows the circuit for measuring open loop characteristics.
Figure Q-33. Circuit for Measuring Open Loop Characteristics
(3) The gain is the output voltage (V1
O
) divided by the differential input voltage (V
id
). The gain is
listed at being infinite and is also negative, indicating inversion. This is an ideal value and will be much
less because of supply voltages. An average value for gain is 5,000 to 100,000 times. This still leads
one to think of tremendous output. To clear this point up, imagine a supply voltage of ± 20V. The output
can only be 40V P-P and the V
id
levels would have to be very small because of such a large gain.
(4) Output offset voltage (V
OO
) is the output voltage to ground or common when V
id
= 0V. At this
time, there is no difference between the two inputs so there should be no output, V
OO
= 0V. In practice
there is always some offset voltage; this is not very serious because external components can be added
to correct the offset voltage to the 0 state.
(5) This 0V offset voltage state is referred to as common mode rejection (CMR). Ideally, CMR
would be 0V but in practice there is some common mode (CM) voltage at the output even though there is
no difference at the input.