Page 92 - Troubleshooting Analog Circuits

More Beta-More Better? 79

beta rises, so does h&. As beta rises and h, rises, the transistor’s output impedance
decreases; its Early voltage falls; its voltage gain decreases; and its common-emitter
breakdown voltage, BVcE~, may also decrease. (The Early voltage of a transistor is
the amount of V,, that causes the collector current to increase to approximately two
times its low-voltage value, assuming a constant base drive. VEarly is approximately
equal to 26 mV X (l/h&)). So, in many circuits there is a point where higher beta
simply makes the gain lower, not higher.
Another way to effectively increase “beta” is to use the Darlington connection: but
the voltage gain and noise may degrade, the response may get flaky, and the base
current may decrease only slightly. When I was a kid engineer, I studied the ways
that Tektronix made good use of the tubes and transistors in their mainframes and
plug-ins. Those engineers didn’t use many Darlingtons. To this day, I keep learning
more and more reasons not to use Darlingtons or cascaded followers. For many years.
it’s been more important (in most circuits) to have matched betas than to have sky-
high betas. You can match betas yourself, or you can buy monolithic dual matched
transistors like the LM394, or you can buy four or five matched transistors on one
monolithic substrate, such as an LM3045 or LM3086 monolithic transistor array.
One of the nice things about bipolar transistors is that their transconductance, g”,.
is quite predictable. At room temperature, g, = 38.6 X IC (This is much more consis-
tent than the forward conductance of diodes, as mentioned in the previous chapter.)
Since the voltage gain is defined as Av = g, X ZL, computing it is often a trivial task.
You may have to adjust this simple equation in certain cases. For instance, if you
include an emitter-degeneration resistor, Re, the effective transconductance falls to
l/(Re + g,-’). Av is also influenced by temperature changes, bias shifts in the emitter
current, hidden impedances in parallel with the load, and the finite output impedance
of the transistor. Remember-higher beta devices can have nzuch worse output
impedance than normal.
Also be aware that although the transconductance of a well-biassed bipolar tran-
sistor is quite predictable, beta usually has a wide range and is not nearly as
predictable. So you have to watch out for adverse shifts in performance if the beta
gets too low or too high and causes shifts in your operating points and biases.

Field Effect Transistors

For a given operating current, field-effect transistors normally have much poorer
g, than bipolar transistors do. You’ll have to measure your devices to see how much
lower. Additionally, the V,, of FETs can cover a very wide range, thus making them
harder to bias than bipolars.
JFETs (Junction Field-Effect Transistors) became popular 20 years ago because
you could use them to make analog switches with resistances of 30 R and lower.
JFETs also help make good op amps with lower input currents than bipolar devices,
at least at moderate or cool temperatures. The BiFET’” process’ made it feasible to
make JFETs along with bipolars on a monolithic circuit. It’s true that the character-
istic of the best BiFET inputs are still slightly inferior to the best bipolar ones in
terms of V,, temperature coefficient, long-term stability, and voltage noise. But
these BiFET characteristics keep improving because of improved processing and
innovative circuit design. As a result, BiFETs are quite close to bipolar transistors in
terms of voltage accuracy. and offer the advantage of low input currents. at room
temperature.

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