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I have created this simple single stage, transistor audio amplifier using voltage divider bias.

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For testing, the source of input signal was audio from laptop and the output was from a 4 ohm, 0.25 watt speaker. When I applied the input signal, I could hear only very very minute sound in the output. When I connected the speaker directly to the source, it produced good audible sound. Why is this amplifier not amplifying then? Where lies the mistake. I've also went through the datasheet of the transistor and suitable chosen the collector current value of 5 milli-ampere and calculated the resistors appropriately. Please suggest some corrections.

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    \$\begingroup\$ Assume, as a first approximation, that the output impedance is 470 ohms, with a signal swing of 9 volts pk-pk. Then what do you think the voltage across your 4 ohm speaker will be? \$\endgroup\$ – WhatRoughBeast Jan 20 '16 at 4:30
  • \$\begingroup\$ i dont understand it. \$\endgroup\$ – Soumya Sambeet Mohapatra Jan 20 '16 at 4:47
  • \$\begingroup\$ Max power is when Load = Source resistance... Try it with a 4 Ohm resistor for RC... And get a bigger transistor. \$\endgroup\$ – MadHatter Jan 20 '16 at 4:48
  • \$\begingroup\$ Did you check the load capacitance and coupling capacitance value.This will affect the bandwidth. \$\endgroup\$ – Aadarsh Jan 20 '16 at 4:55
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The problem is that the amplifier does not have a low enough output resistance to drive a 4-ohm speaker. You're effectively lowering the output gain significantly.

You will need a buffer amplifier after your gain amplifier to drive the low impedance. You could add a common-collector stage to prevent loading down the voltage amplification.

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