_____________________________________________________________________ Special Amplifiers
Figure 7-11. Block Diagram of an Operational Amplifier
7-51. The second stage is a high-gain voltage amplifier. This stage may be made from
several transistors to provide high gain. A typical operational amplifier could have a
voltage gain of 200, 000. Most of this gain comes from the voltage amplifier stage.
7-52. The final stage of the operational amplifier is an output amplifier. The output
amplifier provides low output impedance. The actual circuit used could be an emitter
follower. The output stage should allow the operational amplifier to deliver several
milliamperes to a load.
7-53. Notice that the operational amplifier has a positive power supply (+VCC) and a
negative power supply (-VEE). This arrangement enables the operational amplifier to
produce either a positive or a negative output.
7-54. The two input terminals are labeled' 'inverting input" (-) and "noninverting input"
(+). The operational amplifier can be used with three different input conditions (modes).
With differential inputs (first mode), both input terminals are used and two input signals
that are 180 degrees out of phase with each other are used. This produces an output signal
that is in phase with the signal on the noninverting input. If the noninverting input is
grounded and a signal is applied to the inverting input (second mode), the output signal
will be 180 degrees out of phase with the input signal (and one-half the amplitude of the
first mode output). If the inverting input is grounded and a signal is applied to the
noninverting input (third mode), the output signal will be in phase with the input signal
(and one-half the amplitude of the first mode output).
CLOSED-LOOP OPERATION OF AN OPERATIONAL AMPLIFIER
7-55. Operational amplifiers can have either a closed-loop operation or an open-loop
operation. The operation (closed-loop or open-loop) is determined by whether or not
feedback is used. Without feedback the operational amplifier has an open-loop operation.
This open-loop operation is practical only when the operational amplifier is used as a
comparator (a circuit which compares two input signals or compares an input signal to
some fixed level of voltage). As an amplifier, the open-loop operation is not practical
because the very high gain of the operational amplifier creates poor stability. Noise and
23 June 2005