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Seven- Step
Class A
Transistor Ampli er Design
Edward A. Lawren ce, WASSWD/6
218 Haloid
Ridgecrest , California 93555
•••
II
Design ing a Class A t ra nsistor amplifier is
nol so hard if you are willing to make a few
reasonable assu m ptio ns. As a matter of fact,
it is just an exercise in O h m's Law. The
biggest assu m ption is that the transistor has
u reasonably high gain, and a reasonably lo w
leakage. T his procedure works for NPN,
PNP. Silicon a nd Germ anium. You don't
care if it is Ge o r Si until the 4th step, and if
it is NPN or PNP until the last.
If you need a particular gain (voltage
gain), you can determine the proper values
to get it, o r if you want all the gain you can
get. lake the sa me pro cedure, but bypass the
emitter resistor. Refer to Fig. t for the seven
ste ps and an example to show ho w they are
used .
Once you have designed the amplifier and
asse m bled it, check Ye. It should be about
one-half tile supply voltage. If it isn' t,
c ha nge R B2. Increase it if Yc is too lo w, or
vice versa. If you build for a set gain,
remember to allow for the loading of the
nex t stage by figuring the load in parallel
with RL in step 2. If you decid ed to go all
out fo r gain. pick CE to have a reactance of
about one-tenth the value of RE at the
lowest frequen cy you plan to pass through
the amplifier.
This proced ure will allow you to design a
workable amplifier for almost all applications
for Class A RC coupled amplifiers. You
may come upo n a special case, but I have
developed this pro cedure while working in
various Engineering Departments during the
last three years, so I sincerely doubt it.
Additional Remarks
St ep I : To be able to get the maximu m
voltage sw ing o ut befo re clipping, the collector
needs to be set o ne-half the effec tive
su pply voltage below supply voltage. To find
F ig. 1
1. Pic k RL (.6-20 Kl IC=.5VS/R L
2. Calcu late RE (Gain 1 ·2 5 ) AE =RLlGain
3. Calculate VE (IE about equal to IC)
VE=IC(RE)
4 . Add VBE to VE t o get VB. VBE=.3v for Ge, .6
f or S i VB=VE + VBE
5. P ic k A B 1 (3.3-27 Kl
6. Calculate R B2
RB2=RB 1(VS·VB/VB)
7. If NPN, VS is Positive
I f PNP. VS is Negative
Example: NPN Si, 1 2 V D C . Gai n o f 10
1. R L = 10K. t C= 6 / 1 0 K = .6 ma
2. RE = 10K/ 10= 1K
3 . VE "'.6x 10-3x 1xl03 = .6VDC
4. VB=.6+.6:1 .2VOC
5 . RB1 = 12K
6. R B 2 i n K o hms
RB2= 12( 12-1.2/1.2)= 12(9) : 108, use 11 0 K
7 . VS is Positive
effective supply voltage, subtract YE from
VS. Then drop half of that across RL.
Step 2: This step presumes that the gain
of the transist or is higher than the gain the
circu it asks for. No rma lly this will be t il e
case. If you want all the gain you can get.
bypass the emitter resistor. This will increase
the d istortion somewhat. Usually the distortion
will still be low enough fo r amat eur
purposes, but not lo w enough for " Hi-F!".
Step 3: Since the base current is small
compared to the collector current. this is a
good approximation. It is not advisable to
ground the emitter d irectly, as this reduces
the d e stabili ty grea tly. It also makes t his
procedure almost useless, since some of the
assumptions no longer hold. And one resistor
and a capacitor arc a very small price
to pay for the advantages gained. Also, as a
rule, the more vo ltage you d rop across the
emitter resist or, the more stable the circuit
will be with temperature changes.
72
73 MAGAZINE