Next: Experiment
Up: Op-Amp Basic Concepts
Previous: Small Signal Output and
In this section we show how feedback can be used in
op-amp-type structures to control and improve
circuit performance. The concepts introduced here
form some of the basic ideas for op-amp design.
The circuit in Fig. 5.5 feeds back some of the output
to the inverting input terminal of the diff-amp at the base
of Q2. In this circuit we only feedback the AC part because the DC levels at the
emitter of Q4 and the base of Q2 are very different due to
biasing
For now this lab let's consider only midband operating frequencies
where the feedback capacitor is a short for AC and open for DC.
The effect of the feedback on small signal gain can be illustrated
by the following analysis.
We start with the basic open loop gain definition for our circuit,
and revise it slightly to account for two inputs (we do not ground
the base of Q2).
| |
(130) |
Where Adm is the differential mode gain given by equation
(5.25), and v2 is the small signal at the base of Q2.
Our goal for the analysis will be to eliminate v2 from the expression,
and then determine the closed loop gain
.To keep things simple, and illustrate the main points, we will neglect
loading effects in our analysis.
(This is a good approximation as long as the input resistance
into the diff-amp is much greater than the effective resistance
of the feedback network, and the resistance of the feedback resistors
seen at the emitter of Q4 is much greater than the open loop output resistance
of the circuit.)
After making these approximations, we can immediately express v2
in terms of vo using the expression for a voltage divider
and treating the capacitor as an AC short to obtain:
| |
(131) |
Substituting equation (5.32) for v2 in equation (5.31),
and re-arranging gives the following expression for the closed
loop gain.
| |
(132) |
One very interesting aspect of equation (5.33)
is that if
| |
(133) |
then equation (5.33) can be approximated as
| |
(134) |
This expression should be recognized as that describing
the gain of a noninverting amplifier op-amp circuit.
In the final experiment of this lab, you will construct
such a feedback circuit, and help determine when
equation (5.35) is applicable.
Next: Experiment
Up: Op-Amp Basic Concepts
Previous: Small Signal Output and
Neil Goldsman
10/23/1998