I have built the following circuit intended to be an amplifier. circuit

When I hook the input up to my phone via a guitar cable and the output to a 4 ohm speaker it produces a lower volume than plugging the phone straight into the speakers. The voltages I measure over the resistors do not match up to the tutorial I followed. I have tried switching the transistor and capacitors to no avail. My battery measures at 8.8V.

  • 2
    \$\begingroup\$ Compare the impedance of your speaker to the output impedance of that circuit. \$\endgroup\$ – PlasmaHH Jun 13 '17 at 15:19
  • \$\begingroup\$ I don't have an oscilloscope is there still a way to do this? \$\endgroup\$ – Shibby Shtank Jun 13 '17 at 15:22
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    \$\begingroup\$ you could try maths. \$\endgroup\$ – PlasmaHH Jun 13 '17 at 15:23
  • \$\begingroup\$ Is the point that they're orders of magnitude different or do you really want to know? \$\endgroup\$ – Shibby Shtank Jun 13 '17 at 15:32
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    \$\begingroup\$ You designed an amplifier, but you also need a power stage to drive your load, add an emitter-follower before C2 (which also looks to be reverse biased). \$\endgroup\$ – sstobbe Jun 13 '17 at 15:40

The output impedance of a phone is <10 ohms (it easily drives 8 ohm earbuds), so you're increasing the output impedance with your "amplifier": At the beginning, you bias it using a voltage divider with impedance of around 2kOhms, then at the end of the circuit your amplifier is based on a 4.7kOhm resistor, so it has output impedance of 4.7kOhms. If you're driving a 4 ohm speaker, you have far too high output impedance.

  • \$\begingroup\$ Could this be solved by using smaller values on the resistors? \$\endgroup\$ – Shibby Shtank Jun 13 '17 at 15:31
  • \$\begingroup\$ Probably not. I'd recommend an op-amp amplifier with a push-pull follower, ask me if you'd like me to show that in my answer. \$\endgroup\$ – Ben G. Jun 13 '17 at 15:34
  • \$\begingroup\$ I might be back when I get my hands on an op-amp. Thanks for the help. \$\endgroup\$ – Shibby Shtank Jun 13 '17 at 15:40
  • \$\begingroup\$ @ShibbyShtank: Before you try the op-amp you need to do some calculations to understand what voltages and currents your speaker will require. Then you need to choose an amplifier to suit by checking specifications and datasheets. \$\endgroup\$ – Transistor Jun 13 '17 at 17:31

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