# Designing a Class AB/Class B power amplifier

Discussion in 'The Projects Forum' started by rahdirs, May 29, 2013.

1. ### rahdirs Thread Starter New Member

May 22, 2013
28
0
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

File size:
69.4 KB
Views:
73
2. ### crutschow Expert

Mar 14, 2008
13,024
3,236
Post the rest of the circuit.

3. ### wayneh Expert

Sep 9, 2010
12,145
3,054
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.

4. ### #12 Expert

Nov 30, 2010
16,316
6,818
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.

• ###### BD244C.pdf
File size:
147 KB
Views:
20
Last edited: May 29, 2013
5. ### rahdirs Thread Starter New Member

May 22, 2013
28
0
• 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

File size:
118.1 KB
Views:
37
• ###### Captre.JPG
File size:
87.3 KB
Views:
29
Last edited: May 31, 2013