Unformatted text preview: pli er con guration. 6.4 Departures from Ideal It is no surprise that the golden rules are not exact. On the other hand, they generally
describe most, if not all, observed op-amp behavior. Here are some departures from ideal
O set voltage, VOS. Recall that the input of the op-amp is a di erential pair. If the
two transistors are not perfectly matched, an o set will show up as a non-zero DC
o set at the output. As you found in Lab 4, this can be zeroed externally. This o set
adjustment amounts to changing the ratio of currents coming from the emitters of the
two input transistors.
Bias current, Ibias. The transistor inputs actually do draw some current, regardless
of golden rule 2. Those which use bipolar input transistors e.g. the 741 draw more
current than those which use FETs e.g. the 411. The bias current is de ned to be
the average of the currents of the two inputs.
O set current, IOS. This is the di erence between the input bias currents. Each bias
current, after passing through an input resitive network, will e ectively o er a voltage
to the op-amp input. Therefore, an o set of the two currents will show up as a voltage
o set at the output. 37 Perhaps the best way to beat these efects, if they are a problem for a particular application, is to choose op-amps which have good speci cations. For example, IOS can be a
problem for bi-polar designs, in which case choosing a design with FET inputs will usually
solve the problem. However, if one has to deal with this, it is good to know what to do. Figure 32 shows how this might be accomplished. Without the 10 k resistors, this represents
a non-inverting ampli er with voltage gain of 1 + 105=102 1000. The modi ed design in
the gure gives a DC path from ground to the op-amp inputs which are aproximately equal
in resistance 10 k , while maintaining the same gain.
+ IN OUT - 10k
10k 100k 100 Figure 32: Non-inverting ampli er designed to minimize e ect of IOS.
Similarly, the inverting ampli er con guration can be modi ed to mit...
View Full Document
- Fall '12
- Cybernetics, Positive feedback, de nition Zin