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61
4 Bipolar transistors
whereas, when the transistor is off, the current through it is essentially
zero.
$ When the transistor is on, what are the base and collector currents? (You
don’t need to bother with an ammeter for this measurement – just look
at the voltage drops across the base and collector resistors.)
$ What is your transistor’s saturation voltage VCE(sat) ? What is the ‘on
voltage’ across the LED?
$ Approximately what minimum value of β must the transistor have to be
sure of saturating when +5 V is applied at the input?
$ Drive the switch from the ‘TTL’ output of the function generator (‘digital’
square-wave with a low voltage level near zero and a high level near
+5 V) at 100 kHz and use the dual-trace oscilloscope to measure the
turn-on and turn-off delays in nanoseconds. (Trigger the scope with the
function generator while looking at both the function generator signal
and the collector voltage.)
The relatively slow turn-on and turn-off delays of the saturated switch are
due to the charge stored in the base when the transistor saturates. It takes
time for the transistor to switch states since this saturation charge must
flow in or out of the base through the input resistor. High-speed switching
transistors (such as the 2N2369) are manufactured to minimize this effect
and can operate at frequencies as high as 1400 MHz.
4.3 Additional exercises
The following optional exercises offer additional practice.
4.3.1 Darlington connection
To provide high input impedance and reduce the input base current, one
can cascade two transistors in series, i.e. ‘buffer’ the input with an emitterfollower stage. This Darlington transistor pair acts like a single transistor
whose current gain is the product of the two β’s and whose VBE drop is the
sum of the two VBE ’s. Build the circuit of Fig. 4.10.
$ With the input grounded, what quiescent currents do you observe through
RB and RE ? What does this imply for the combined β value of the
Darlington pair?
$ Now apply an input signal – what do you see at the output? What is the
DC voltage drop from input to output?