# Speaker BJT Amplifier - very low gain

First of all this is not related to a HW or anything; I just wanted to build an audio amplifier and test my skills. I really don't see why my design is not working as intended, without the speaker load it amplifies the input signal just right but when I connect the speaker load (4 Ohm) the power of the speaker is limited to microwatts. Can anyone help? Sorry I didn't sketch the circuit in the editor since I already had drawn it in LTspice P.S. I actually have built the circuit but I can barely hear something coming from the speaker when I blow on the mic.

• Your values for R4 and R5, especially R5, limit the current available to the low resistance of the load, so the load power is very low. Ignoring C2 for a moment, the maximum voltage across the speaker is 24 x (4/2204), or about 44 mV, for a maximum peak power of 47 uW. Jun 16 '16 at 15:24
• Please space out your schematic, it's very hard to read. Also, you might consider linking us the LTSpice file so we can verify your simulation. Finally, the 1 mF capacitors look suspicious, are those the intended value? Jun 16 '16 at 15:25
• This circuit cannot be used as a POWER amplifier. Its a voltage amplifier but it has virtually no current driving capability. Connecting a 4 ohm speaker at the final stage is like adding a short circuit to the output. Jun 16 '16 at 16:15
• Did you even look at any audio power amplifier design before you started? Jun 16 '16 at 16:50
• Suggested reading: Nelson Pass Zen amplifier, or Douglas Self's various writings on audio amplifiers. Your output stage is not practical for a power amplifier. All the current that goes to the speaker has to pass through R5. Jun 16 '16 at 16:54

The gain of the last stage is approximately equal to: $$Gain_3 = {{R8}\over{R4}} = {{4}\over{2200}}=0.001818$$ You must use a push-pull emitter follower. Transistors desirable to take a large gain. With such a supply voltage using two stage voltage gain enough ($Gain=1000 \div 5000$) output and push-pull emitter follower (gain approximately equal 1). The last stage can be done on a single-ended power transistor with the radiator cooling. But modal current in this case is very high.