Lab 5: Bipolar Transistors (BJTs)
Reading Assignment
1. HH Lab Manual Class 4 and Worked Examples (pg 84-93 of lab manual)
2. HH Text pg 61-79
3. Hands-On Ch 4-4.1
Additional background:
A. Variable Resistors and Potentiometer, or POTs
Source: http://www.allaboutcircuits.com/vol_1/chpt_6/1.html, http://sound.westhost.com/pots.htm
One device frequently used as a voltage-dividing component is the
potentiometer, which is a resistor with a movable element positioned by a manual
knob or lever. The movable element, typically called a wiper, makes contact with
a resistive strip of material (commonly called the slidewire if made of resistive
metal wire) at any point selected by the manual control:
3
The wiper contact is the left-facing arrow symbol drawn in the middle of the
vertical resistor element. As it is moved up, it contacts the resistive strip closer to
terminal 1 and further away from terminal 2, lowering resistance to terminal 1 and
raising resistance to terminal 2. As it is moved down, the opposite effect results.
The resistance as measured between terminals 1 and 2 is constant for any wiper
position.
Some linear potentiometers are actuated by straight-line motion of a lever or slide
button. Others are actuated by a turn-screw for fine adjustment ability. The latter
units are sometimes referred to as trimpots, because they work well for
applications requiring a variable resistance to be trimmed to some precise value.
Being 3 terminal devices, there are quite a few different ways that they can be
wired. A pot is usually wired using all three terminals. One terminal (2) is
earthed (grounded) for use as a volume control - the most common usage. This
allows the wiper to be turned all the way to zero signal for maximum attenuation.
Note that if the earth terminal were to be left disconnected, all we have is a
variable series resistance, whose effectiveness will be minimal in typical circuitry.
Turning the shaft clockwise (CW - by convention, to move pin 3 physically closer
to pin 1, and increase (for example) volume) will select a different point along the
resistance element, and forms a voltage divider, so the attenuation of the signal
is proportional to the rotation of the shaft. At the fully clockwise position, there is
close to zero ohms in series with the signal, and the full resistance of the pot to
earth. Attenuation at this setting is zero (assuming a zero or low impedance
source - this is often overlooked!), and this is full volume (maximum signal level).
The source impedance should normally be no greater than 1/10th (0.1) of the
pot's stated resistance. Further, the load resistance or impedance should be 10
times the pot's resistance to prevent the taper (law of how a trimpot works =
linear or log) from being adversely affected.
B. Transistors as saturated switches
Source: Eyler, lab 5
The use of transistors as for logic and power control usually depends not so
much on their linear gain characteristics as on their ability to turn on and off
rapidly. For this purpose the base-emitter junction is switched between two
states: completely cut off or strongly forward-biased. The result is that the
collector-emitter voltage VCE is either equal to the full supply voltage or to a very
low saturation voltage, with a voltage drop that may be less than 0.1 V. In this
laboratory we will look at a saturated switch at low speeds. Very fast switching
involves special difficulties, some of which are discussed in Chapter 13 of
Horowitz and Hill.
As mentioned in Appendix G of Horowitz and Hill, the saturation voltage can
never actually reach zero, because for very low values of VCE the base-collector
junction becomes forward-biased, and more and more base current is needed to
further reduce the collector-emitter drop. The effective current gain becomes
very low, and can be orders of magnitude less than the small-signal hFE. Often
it’s worth supplying this excess base current even though the external voltages
are hardly affected, because if the transistor is switching a high-current load, the
power I ×VCE dissipated in the device can get very large. The figure below shows
the saturation curves for the 2N4400, showing the dramatic dependence on
collector current.