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RAY MARSTON
B I P O L A R J U N C T I O N T R A N S I S T O R (BJT) amplifiers are still widely used in modern electronic cir- cuitry. This article focuses on practical variations of the com- mon-collector or erniiter-fol- lower arnpiifier based on dis- c r e t e t r a n s i s t o r s a n d Darlington pairs. Figure. 1 shows the basic comrnon-col- lector amplifier and compares it with the cornmora-base a n d common-emitter amplifiers. Ta- ble I sums up the perfol-mance characteristics of these three bi- polar amplifiers. The fundamentals bipolar of trar-~sistorswere presented last month and the specifications of two widely available and typical d i s c r e t e devices, t h e N P N
2N3904-and the PNP 2N
were given. The 2N3904 is in- cluded in most of the sche- matics in this article. The expression h,, in Table 1, known as a hybrid parameter, is the common-emitter DC for- ward-current gain. I t is equal to. the collector current divided by the base current (h,, = I,II,). The value of this variable for the
2N3904 NPN transistor is typ-
ically between 100 and 300, but in this article it is considered to be 200. A lot of useful information can be gained simply by study- Ing both Fig. h and Table 1. The common-c~llectoramplifier (also widely known as the emit- ter@llowes) has its input ap- plied between i t s base a n d collector and its output is taken
across its emitter and coliector. The circuit is also referred to as the grounded-collector ampli- fier. In practical configurations its load resistor is in series with its emitter terminal. The mathematical deriva- tions of the results shown in Ta- ble 1 can be found in most basic electronics texts. However, for the purposes of this article. the 8 important characteristics of the $ common-collectorlemitter fol-
lower amplifier to keep in mind 8
are: @ High input impedance (D 2 a+ Low output impedance 5 2.
@ Voltage gain approsinlately g
equal to unity z
0 @ Current gain approximately i equal to h,,
By contrast, notice that while 57
FIG. 1-THREE BASIC BIPOLAR tran- sistor amplifier configurations.
the common-emitter and com- mon-base amplifiers provide high voltage gain they offer only low-to medium input imped- ance. The applications for these circuits are governed by these characteristics.
Digital amplifiers
Figure 2 is the schematic for a simple NPN common-collector/ emitter-follower digital ampli- fier. The input signal for this cir- cuit is a pulse t h a t swings between zero volts a n d the positive supply voltage. When the input of this circuit is at zero volts and the transistor is fully cut off, and the amplifier's output is also zero volts-indi- cating zero voltage phase shift. When a n input voltage ex- $ ceeding^ +^ 600 millivolts (the 8 m i n i m u m forward b i a s for turn-on) appears across the in-
- put terminals, the transistor
,O turns on and current I, flows in
.-^ yj^ load resistor^ R,,^ generating an
output voltage across R,. Inher-
$ ent negative feedback causes 0) the output voltage to assume a value that follows the input 58 voltage. The output voltage is
TABLE 1
CHARACTERISTICS OF THE THREE BASIC TRAMSBSBOR AMPklFBEWS
Common Common
Collector Emitter Base
Input impedance ZIN High Medium Low ( = ~ F E X RL) ( = I .OK) (=401E) Output impedance , ,Z, Very low =R, =R, Voltage gain A, = 1 High High Current gain A 1 = ~ F E " ~ F E - 1 Cutoff Frequency - Medium Low High Voltage phase shift. -^^0 "^ 180"^ 0"
FIG. 2-COMMON-COLLECTOR digital amplifier.
FIG. 3-EFFECT OF CAPACITOR Cs on output pulses.
driver.
e q u a l to t h e i n p u t voltage minus the voltage drop across the base-emitter junction ( - 600 millivolts). In the Fig. 2 schematic, the input (base) current is calcu- lated as: IB = I L b E
Because the circuit can have a maximum voltage gain of one, it presents a n input impedance calculated as:
Z,, = R, x h,,
Inserting the values shown in
Fig. 2 yields: Z,, = 3300 ohms
X 200 = 660,000 ohms The circuit has an output jm- pedance t h a t approximately equals the value of the input signal source impedance (R,) divided by the h,, value of the transistor. Because the circuit shown in Fig. 3 exhibits all of the com- mon-collector amplifier charac- teristics previously discussed,it
behaves like a unity-gain bufler
c i r c u i t. I f h i g h - f r e q u e n c y
pulses are introduced at its in- put, the trailing edge of the out- put pulse will show the time constant decay curve shown in Fig. 3. This response is caused by stray capacitance C , (repre- sented by dotted lines) interact- ing with t h e circuit's load resistance. When the leading edge of the input puise switches high, Q
switches on and rapidly sources
or feeds a charge current to stray capacitance C,, thus pro- ducing an output pulse with a sharp leading edge. However, when the trailing edge of the in- put pulse goes low, Q 91 switches off and effective capacitor C , is
unable to discharge or sink
through the transistor. However, C, can discharge
through load resistor R.. That
dischzrge will follow a; expo- nential decay curve with the time to discharge to the 3 7 % level equal to the product of C ,
and R,.
Relay drivers
The basic digital or switching
rent generator.
stant-current generator with a LEQ volt- age reference.
FIG. 13-SIMPLE EMITTER-FOLLOWER.
7 and 8, operating delays will be about 0.1 second per micro- farad of capacitor value. For ex- ample, if C1 equals 100 micro- farads, the time delay will be 10 seconds. In the Fig. 7 circuit, consider that C1 is fully discharged so that the Rl-C1 junction is at zero volts and relay RY1 is off (contacts open) when the power supply is connected. Capacitor C1 then charges exponentially
through R1, and the increasing voltage is fed to the relay circuit through Darlington pair Q1 and Q2. That causes relay RYlb con- tacts to close after a time delay determined by the product of R and C1. Consider that capacitor C1 in the Fig. 8 circuit is also fully discharged when the power supply is connected. The junc- tion of R1 and C1 is initially at the supply voltage, and the relay contact close at that moment. Capacitor C1 then charges ex- ponentially through R1, and the decaying voltage at the R1-C junction appears across the coil of relay RY1. The contacts of RY open after the delay determined by R1 and Cl times out.
Constant-currentgenerators
A BJT can serve as a con- stant-current generator if it is connected in the common-col- lector topology and the power supply and collector terminals function as a constant-current path, as shown in Fig 9. The 1000-ohm resistor R2 is the emitter load. The series com- bination of resistor R1 a n d Zener diode Dl applies a fixed 5.6-volt reference to the base of
There is a 600-millivolt base-
to-emitter drop across Q1, so 5
volts is developed across emitter resistor R2. As a result, a fixed current of 5 milliamperes flows through this resistor from Ql's emitter. Because of a BJT's charac- teristics, emitter and collector currents are nearly identical. This means that a 5- milliam- pere current also flows in any load that is connected between Ql's collector and the circuits's positive supply This will occur regardless of the load's resis- tance value-provided that the value is not so large that it driv- es Q1 into saturation. There- fore, t h e s e two points a r e constant-current source termi- nals. Based on the previous discus- sion, it can be seen that con- stant-current magnitude is de- termined by the values of the base reference voltage and emit- ter load resistor R2. Con- sequently, the value of the
current can be changed by vary- ing either of these parameters. The Fig. 30 circuit takes this concept a step further. It can be seen, %orexakple, that the cir-
cuit of Fig. 9 was inverted to
give a ground-referenced, con- stant-current output. Adj.ust- ment of trimmer potentiometer R 3 provides a current range s f from 3 to about BO milliamperes, The most lmportant feature of the constant-current circuit is its high dynamic output im- pedance-typically hundreds of
FIG. 14-HIGH-STABILgT'b EMITTER-301- lower.
follower.
FIG. 16-BOOTSTRAPPED Darlington emitter-follower.
FIG. 17-B80T%TRAPPED @omplemees- tary feedback pair.
FIG. 18-COMPLEMENTARY EPdITTER- foilower with a split power supply and direct-coupled output load.
10pF i
INPUT ; :$ ‘I
FIG. 19-COMPLEMENTAR\tr EMSTTER- follower with a single-ended supply and AC-coupled output BoaC.
kiiohms. Tk~rprecise magni-
tude of constant current is usu-
aliy unirnpsriant in practical circuits. The circuits shown in
Figs. 10 alrd I 1 will work satis-
factorily ifi many practical ap- plications. If more precise c r l r r e ~ ~ igenel-- ation is required, the cbarac- teristics of the refererace volt- ages of these clrcuirs can be improved to elrmlnate t h e effects oi power source 1Jar:a-
tions and temperature changes. A simple way to improve'the circuits in Figs. 9 and 10 is shown in Fig. 11. Resistor R1 in both circuits can be replaced with a 5-miiliampere constant- current generator. (The symbol for a constant-current gener- ator is a pair of overlapping cir- cles.) With a constant-current generator installed, the current through Zener diode Dl and the voltage across it is independent of variations in the supply volt- age. True high precision can be ob- tained if the industry standard reference zener diode D1 is re- placed with one having a tem- perature coefficient of 2 milli- volts/"C to match the base-to- emitter temperature coefficient of transistor Q1. However. if a zener diode with those charac- teristics cannot be located, sat- isfactory results can be ob- tained by substituting a for- ward-biased light-emitting di- ode, as shown in Fig 12. The voltage drop across LED is about 2 volts, so only about 1.4 volts appears across emitter
resistor R1. If the value of R1 is
reduced from 1000 to 2 7 0 o h m s , the constant-current output level can be maintained at 5 milliamperes.
h d s g amplifiers
The common-collector/emit- ter-follower amplifier can ampli-
x l 0 ~ i Q 2N3906; INPUT
FIG. 20-DWRL16dGTOM complementary emitter-follower with "amplified diode" biasing from transistor Q5.
fy AC-coupled analog signals linearly if the transistor's base is biased to a quiescent value of about half the supply voltage. This permits maximum signal swings without distortion due to clipping. As shown in Figs. 13 and 14, the analog signals are AC-coupled to the base with ca- pacitor C1, and the output sig- nal is taken from the emitter through capacitor C2. Figure 13 shows the simplest analog common-collector/ emit- ter-follower circuit. Transistor Q1 is biased by resistor R1 con- nected between the voltage source and base. The value of resistor R1 must be equal to the input resistance R,, of the emit- ter-follower stage to obtain half- supply biasing. Input resis- tance R,, (and thus the nominal R1 value) equals the 4. 7 K value of R2 multiplied by the h,, val- ue of the Q1. In this circuit:
R , = 2 0 0 x 4 7 0 0 - lmegohm
A slightly more elaborate bias- ing method is shown in Fig. 14. However, its biasing level is in- dependent of variations in tran- sistor Ql's h,, value. Resistors R1 and R2 function as a voltage divider that applies a quiescent half-supply voltage to Ql's base. Ideally the value of R1 should equal the value of R 2 in parallel with R,,. However, the circuit works quite well if resistor R has a low value with respect to
R,,, and resistor R2 is slightiy
larger than R l. In the circuits shown in Figs. 13, and 14, the input imped- ance looking directly into the base of transistor Ql equals h,,
x Z ,,,,,, where Z ,,,, is equal to
the combined parallel imped- ance of R2 and any external load
2 , that is connected to the out-
put.
In these circuits. the base irn- 8
p e d a n c e value is a b o u t 1 o- ?
megohm when 2 , is infinite. In
practical circuits, the input im- 8 pedance of the complete emit- ter-follower circuit equals the combined parallel impedance of a
the base and bias network. The z.
circuit shown in Fig. 13 has an input impedance of about 500 2 kilohms, and the circuit shown
Continued on page 83 61
