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.Design Notes:
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.
Re = 0 - 4.7 Ohm (even 27 Ohm for earphones)
P1 = R2 / 5
P2 = Rfb/ 10
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