Design of Negative Feedback

In Amplifiersn

In Amplifiersn

Article : Lazar Pancic

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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 radios.

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: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)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).

Calculations:

Suitable values for the other components:

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

Setting Up:

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.