I'd suggest you start by choosing a relatively simple 9V circuit of the many 2-transistor circuits available by searching for "microphone amp" from G00gle images. To this you add a simple complementary class AB output stage. Then take the feedback for the 23dB gain from the output.
You may have found a schematic (top left corner) which meets the design specs. Using it unmodified may be the fastest path to completing the project. You'll learn a lot either route you take.
Your load needs several mA going through it in order to meet specifications.
Your load is powered by the output transistor turning mostly on, then mostly off.
This results in reciprocating current flow back and forth through the load. In from the output transistor, then back through resistor R6.
R6 is 10k ohms.
The transistor may be in the correct operating range. It may turn on sufficiently to power your load.
However when the transistor is almost turned off, your load has only that 10K resistor to conduct through. It's impossible for more than a mA to flow during that half of the cycle.
Hence the loss of gain.
It would help if you were to try various values for R6.
Your latest change was to install a load of comparatively low resistance, among components which have comparatively high resistance.
It caused a big change in circuit behavior.
It helps if you adjust components gradually. That way you can spot a bad trend and try something else.
Changing one component often makes it necessary for you to adjust other components as well.
Anyway, just to make this easy...
Change the load to 1k ohm. This will alter behavior but not so drastically that you can't spot what direction the output is going. Change R6 in such a direction as to bring things back to proper operation. Or change the bias current, etc.
Another tip... The 100nF capacitor is too small to pass audio frequencies to a low impedance load.
The problem I'm experiencing with my gain when the load is attached is clearly a loading issue correct?
If this is the case, couldn't I rectify the problem by simply inserting a buffer stage inbetween the coupling capacitor and the load?
Would a simple emitter follower do the trick?
No, for such a low load impedance I'd suggest a class AB output buffer, s. the foll. PDF: View attachment 63708
Some notes:
Good luck! erikl
- You probably must change R6 in order to get ≈VDD/2 = 4.5V at the junction of R10 & R11
- The pre-driver stage (Q2..R6) will have lower gain than before, and the output buffer stage (QN2/QP2) has a gain < 1
- That is why you'll have to change the feedback resistor R5 to restore the necessary gain
All this is progress. A push-pull amplifier is the solution for you to get a lot of decibels while limiting current draw to 20mA
However are you allowed to use 4 transistors according to the design rules?
To get the meter to read V/2, I think you'll have to make Q2's average impedance match the resistance in the corresponding position on the totem pole. This is R6.
To achieve this there's a chance you'll need to adjust a value or two in an earlier stage.
Continue to experiment. You should not just get the circuit operating, but have an idea why the correct component values work correctly, and why other values didn't work.
Yesterday you were flustered. Since then with help from this board you've made progress toward the assingment. The help is given cheerfully and with the aim to encourage. However you need to be able to show the instructor you can do it on your own, just in case he singles you out in front of the class as an exemplary student.
:^)
Actually for your task it is not, as you need only a max. of 500mVss output voltage.... I wasn't able to set R6 such that the voltage at the junction of R10 and R11 is ~4.5V.
Is it crucial that it sits at exactly 4.5V?
This has no effect on gain or output quiescent voltage. These resistors are only useful against output short circuit protection, and to reduce the quiescent current through the output transistors Q3 & Q4. Reasonable values are between 0 and 10Ω , with this latter value you probably push the quiescent current through the driver stage below 1mA (battery lifetime!).I tried ranging R6 from a 10 ohms to 10Mohms ...
Yes, absolutely!Is what I have so far reasonable
I'd suggest to separate the power supply of the electret mic (0.5mA) and the first preamplifier stage (Q1) by an appropriate RC combination from the 9V battery power supply. This is apt to decrease interferences from the output stage, see e.g. the right-most circuit of your circuit collection from your post #3.... is there other improvements that I should make?
I think, there's a misunderstanding of the circuit operation. The output voltage is set by DC feedback and the ratio of R1/R2, that can be adjusted accordingly. R6 in constrast sets the bias current of the output stage. The crossover distortion suggests, that it may need to be increased, there's however a problem of output transistor's limited power rating.To get the meter to read V/2, I think you'll have to make Q2's average impedance match the resistance in the corresponding position on the totem pole. This is R6.
... I must match the average impeadance of Q2 with R6.
What is the average impeadance of a transistor defined as?
You mention that in order to obtain a voltage of 4.5V at the junction of R10 & R11 I must match the average impeadance of Q2 with R6.
What is the average impeadance of a transistor defined as?
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