Transistor AC Common Emitter is not inverting... why?

I'm trying to learn more about transistors by experimenting with some basic building blocks - I thought this one would be simple but I have one odd problem... the output is not inverted.

There are examples here, here and here that clearly state this should be an inverting configuration.

My aim is to use a single transistor to invert an AC 10V (+/-5V) PP signal. From this config I get almost unity gain (less a diode voltage drop?) - but no inversion.

What am I doing wrong/missing?

Thanks.

Check that the base is biassed properly. it looks like its being fed from +12 via a 10K which is far too low. This is being done on some sort of simulator? A lot depends on whether the audio signal generator can conduct DC through the 1k resistor to earth.
Think I want a collector current of... then this current also flows through the emitter resistor and the collector resistor. So is there enough volt drop across the transistor to allow it to work?, if not change current and/or resistors. Gain of circuit = collector resistor/emitter resistor. Once you have settled on some values, calculate Ve (-12 + (re X Ic)). add +.8v for the Vb-e, to find voltage on base then fiddle about with base circuit to arrive at this figure. This is it it will work.
Frank

You get the base-collector diode foward biased by applying unsuitable DC voltage levels to the transistor, and simply copy the input voltage to the output. You can play around with bias voltages and try to make the circuit work as an amplifier. Or refer to known working amplifier circuits.

That's a big improvement - it's inverting now.

Adjusting the bias to Q1 base meant I had to re-adjust the values of Ra and Rb - I think I was originally adjusting these away from inversion.

I added a 220K load to GND (R5) to dampen the gain a little - there's definitely a trade-off between gain and biasing (unity/lower gain seems to give a heavy upward/positive bias and better biasing around 0V seems to result in greater gain).

The one side-effect this config seems to have (didn't notice it on the previous version - but didn't check) - when there is no signal applied to Q1 Base - the output swings high to about 8V. Is that 'normal'?

Thanks - appreciated.

I can't comment the circuit because the unclear bias of the audio in (apparently it's DC coupled) doesn't allow to calculate a bias point. A complete schematic would at least clarify the DC voltage and resistance connected to the input. Similarly, the connected load should be clarified.

I don't see a reasonable purpose of R5. Adjusting the ratio of R3 and R4 and the input bias network should be sufficient. To make the bias point independent of source impedance and possible superimposed DC voltages, small signal audio amplifiers are usually designed with input (and also output) AC coupling. This would also avoid confusion in the present case, I think.

juz_ad, do you mean that with your signal generator connected but turned down, you get +8V? If so this shows that the collector voltage can only swing to +12V, i.e. by +4V. This could limit your voltage swing. For optimum voltage swing:- you have 24V available, you should allow a few volts across the emitter resistor, say 4V, this then gives 20 volts across the transistor, allow another couple of volts across the transistor (to keep it out of saturation), this then gives 18V. So the collector should be sitting at something like +12 - (18/2) = 3V. This would then give the collector the ability to swing +9V to +12 and -9V to -6V. So take another stab at your currents and resistor values to try and bring the collector voltage down, but at least its working!
Frank

for the orginal circuit -- you've saturated the device. heavily at that. as a result, the output will be close to the emitter voltage, which is close to the base voltage, which is close to the input.

there are a few different equations involved. For simplicity, I suggest AC coupling the input and output, as this simplifies the design. You don't have to worry about source impedance affecting the biasing, nor do you have to worry about biasing to get 0V on the output.

also, the output changes with no input connected because the biasing is affected by the source impedance. When a source is connected, the pin is a low impedance connection to an average of 0V. When disconnected, it is high impedance. This changes the bias voltage at the base, and thus the biasing of the output.