Hi all,

I need to design a power amplifier (power consumed is a criterion so did not try class A)that provides a current gain from 25 mA to 10A keeping the voltage constant.The 25 mA is coming from op-amp.

I tried and got only an amplification of 60 as in schematic.We can't change the impedance of stage next to amplifier

 

Post the rest of the circuit.

 

Can't change the impedance? Says who?

You can't drive enough base current into your transistors with that arrangement. Figure you need 10% of the emitter current at the base.

 

Your transistors are only promising to supply 6 amps. You need a current gain of 400 to one. Your transistors are only promising a gain of 15 amps per amp. That means you need to drive the output transistors with 2/3 of an amp. That is definitely a change of impedance.

Here is the datasheet for your transistors. Pick a different pair of transistors if you want this circuit to work properly.

 

• The output of power amplifier is to be connected to charge a battery.The input is coming through an op-amp at 13-14 V(almost constant) & 25 mA.

• To charge battery i need it around 13 V & 10 A. Knowing that BD 223C is weak i used 2N3055A in darlington configuration.

• But after connecting it to rechargeable battery of 12V,150Ahcapacity,5.88mOhm internal resistance as in fig.1 the charging current shot to 42 A & the voltage at the i/p & o/p of power amplifier changed to negative.

• As i/p of power amp is constant at 14 V,the p-n-p transistors & -ve supply are redundant as it always remains off,so i removed them as in figure.2.Now the charging current came down to 22 A but the voltage also fell to 1.5 V.
  So how do i minimize the voltage drop of two base-emitter junctions.

I can limit current using series current sensing resistor