This article describes how to modify a small class AB amplifier for low distortion and
minimal power drain. This is especially important in battery powered equipment such as
A few days ago I bought little FM receiver (8 x 5 x 1.5cm) and was delighted with how
many stations that the little thing can receive. However when I measured its current
consumption I found that it was 22mA when driving two small 32 Ohm earphones. This was
just too much battery drain from only two "AAA" cells. When I opened the case I could see
a Class A negative feedback (NF) amplifier which directly drove the earphones. So I took
two more transistors and rebuilt a class AB amplifier. Now my little receiver only takes
7mA from the batteries (1.5mA for NF amplifier) - 3 times less than before! You see, little
NF amplifiers are easy to build if one can make some calculations - but most electronic
hobbyists do not know how to do that. I find that these things are really useful blocks in
amateur constructions. Below is the modified class AB design that Lazar built.
First of all one must know for which power supply voltage is the amplifier going to be built - that is Vcc;
Then the impedance (resistance) of the loudspeaker or the earphones RLsp ;
Next choose suitable output transistors. They must be able to dissipate at least the maximum power output which is:
Pmax = (Vcc)2 /(4 x RLsp)
This is very important for T3 and T4. They are NPN / PNP complementary transistors and
they usually have very similar current gain. It is wise to buy them together from the same
manufacturer. I prefer BC547C/BC557C for driving the earphones and BC337-40 / BC 327-40
for driving little 8 Ohm loudspeaker (Pmax = 0.8W). T1 and T2 can be of any common low
power and general purpose series, for example BC547, BC109, "9014" - very common in
cheap AM/FM receivers...
It is beneficial to know the current gain of all 4 transistors - B1, B2, B3, and B4.
It is wise to choose transistors with greater gain to favour lower quiescent current.
These ones usually have the suffix "C" in their names (for example: BC547C).
I1 = (1.5 X Vcc) /(B3 x RLsp)
R1 = ((Vcc / 2 ) - Vbe) / I1
R2 = R3 = (Vbe x SQR(B2)) / I1 not critical at all
Rfb = ((Vcc / 2 ) - Vbe) x B1 / I1 x0.95 if there is P2
CLsp >= 1 / (2 x Pi x RLsp x Fmin) Pi = 3.1415926...
Cin >= I1/ (2 x Pi x B1 x Vgm x Fmin) where Fmin is the lowest frequency to be amplified.
Suitable values for the other components:
Re = 0 - 4.7 Ohm (even 27 Ohm for earphones)
P1 = R2 / 5
P2 = Rfb/ 10
The preset, P1 's purpose is to adjust the quiescent current of T3 and T4. First put it's
slider in the highest position (on the picture) and, measuring the quiescent current of T3
and T4, slide it downwards. Be careful !!! That current should be ~ 0.5 - 2 mA for the
transistors I mentioned above. The adjusting process is much easier if R2 and R3
are not "0 Ohm". After the preset, P1 has been adjusted, it should be replaced with two
fixed resistors, equal to the resistance of the preset, from wiper to end contact.The
purpose of P2 is to adjust the output quiescent voltage. It should be set so that half
the supply voltage is measured at the junction between the two emitter resistors, Re
and ground, but small differences are not important. Only the maximal output voltage swing
would be decreased. The amplifier should be working O.K. even without adjusting P2.
Modification to use an Op-Amp:
Some modifications may be applied. If building an amplifier, an operational amplifier may
be put to good use to drive the output power transistors - see the diagram below:
R1, R2,3 and CLsp may be calculated as described previously.
A = R5/R6 where A is the overall voltage gain.
Choose the resistor values as you wish!
Cin >= 1 / (2 x PI x R6 x Fmin)
R7 = R8 = 10 - 100kOhm. C8 = 1uF
The amplifier in this configuration is much more stable and with much lower distortions!
With these calculations the NF AB-class amplifiers for Pout <= 1W can be made. For more
powerful amplifiers it is better to find some specific schematics.