Basic Common Emitter Amp Help

[Obe1]

Hello All,

I breadboarded a very basic Common Emitter voltage gain stage as seen in the diagram below and the results are confusing to say the least.

The steady state (Q point) voltage readings (shown in red) seem reasonable, yet this circuit does not behave as I would expect when an audio (AC) signal is applied to the input jack. For example, when I plug an MP3 player into the input jack with a signal level of approximately 0.050 mV AC RMS, the output jack signal measures only about 0.040 mv AC RMS. (Note that these signal levels were measured with nothing more than a cheap Digital Multi-meter).

While I don't have a real signal generator or oscilloscope to run accurate tests, when I connect the output jack of this circuit to an auxiliary input on my Yamaha integrated SS amplifier, the results I hear through my loudspeakers seem to verify what I can read off my DMM. That is to say that the sound level out of the loudspeakers when the Common Emitter stage is in the signal path is less than when I connect the MP3 player directly into the Yamaha.

I'm trying to learn about basic amplifier circuits by trying this very crude circuit as a first step and I realize that it's biasing technique and high output impedance are far from ideal, but a guy's got to start somewhere.

Any feedback on why I'm seeing what I'm seeing will be much appreciated.

[Obe1]

Originally Posted by Obe1

For example, when I plug an MP3 player into the input jack with a signal level of approximately 0.050 mV AC RMS, the output jack signal measures only about 0.040 mv AC RMS. appreciated.

This should read as 50 mV on input jack and 40 mV on output jack.

Sorry about that.

[AndrewT]

Does R2 convert this to a unity gain amplifier?

[Obe1]

Hello Andrew,

Well this circuit sure acts more like a buffer than an amplifier. If I understand the theory correctly, R2 does provide bias to the transistor as well as some negative feedback for thermal stability, yet not so much feedback as to negate any decent voltage gain. I was expecting to see a gain equal to approximately - R3/R4 (-10K/1K = -10)

Just for the heck of it I removed R2 and replaced it with a more traditional bias circuit as seen in the diagram below. As expected, the steady state DC voltages remained about the same as before. Unfortunately, the final results were the same when I applied an audio signal to the input as well (ie. no output voltage gain, just a bit of attenuation instead).

I'm so confused!

[Mooly]

I think you will find the input impedance is low and that will reduce the level from the source as you connect it.

[Mooly]

Also what is the load it is working into ? as any significant loading at the collector will reduce the gain.
Your circuit and basic theory is sound... you really need a 'scope etc to see for real what and why it behaves as it does.

[Obe1]

Hello Mooly,

Yes, a scope would be nice. Someday maybe.

Yes, I expected impedance matching to be an issue. In fact, when I started out I had a second 2N2222A wired up as an emitter-follower in the circuit and the performance was the same. So I decided to remove the emitter-follower portion to isolate the problem that I'm seeing now with this simplified circuit.

The audio source is a little Sansa MP3 player that normally drives cheap headphones just fine. The input impedance to my Yamaha amp is specified as being 47K Ohms.

As I said in the beginning, I'm not expecting stellar performance from this experiment, but I was hoping to see some modest amount of voltage gain from my little common-emitter circuit. It seems like I need to sort this out before I explore more sophisticated audio circuits.

[runebivrin]

If it measures OK from a DC perspective, there's a fair chance you've switched collector and emitter.

[Obe1]

Hello runebivrin,

Yes!!!!!!!! Thank you!!!!!!!!!

I really thought I had checked the pin assignments correctly, but obviously I did not. How embarrassing. I must now go and wipe a 1000 Kilograms of egg off my face.

Best regards,

Obe1

[runebivrin]

Don't sweat it. It happens to the best. I know it's happened to some of my best output transistors...