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I am currently trying to build an amplifier for a condenser microphone in order to use it as a way to amplify an acoustic guitar. I am having some trouble to achieve my goals.

In order to learn I decided to make some analog simulations using QUCS-Spice and the Ngspice simulator. I copied a common-emitter Class A amplifier from the internet and I tried to bias the transistor 2N3904 in order to achieve an amplification in current as well as in voltage in the output, which will be a 35 Ω old ear-speaker.

There is something I can't achieve independently of the resistors and capacitors used for the common-emitter Class A amplifier circuit, and would like to ask how I can improve the voltage gain of the circuit, in order to amplify the power gain within it.

In order to explain myself I will post some pictures with two different resistors and their outputs, following Ngspice simulation results.

In order to amplify the signal from a condenser microphone, model KPCM-G97H45P-43DB-1187, using a battery of 9 V as a power supply, I decided to use a resistor for supplying the power to the microphone condenser of 14.7 kΩ in series with the microphone and a 220 μF capacitor in order to decouple the DC signal and send it to the amplifier circuit, correct me if this is not the right verb (decouple).

Using just the microphone within the 9 V power source it is possible to see the signal in audio recording software like Audacity, but the signal is quite low, I can just see like a flat line without any wave looks.

  • Circuit Number 1 - "Normal" Resistor selection

enter image description here

For the first biasing point I used the characteristics plot above from a data-sheet found online.

I chose 0.04 A as closed switch current for the transistor, V_CE=0 and made R_C = 3/4 × R_Total and R_E = 1/4 × R_Total. Therefore:

R_Total = V_CC / I = 9 / 0.04 = 225 Ω
R_C = 3/4 × R_Total = 168.75 Ω
R_E = 1/4 × R_Total = 56.25 Ω

From here I selected a Q-Point of I_Base = 100 μA with approximately a collector current of 0.02 A, following the graph above.

Using I_Collector = 0.02 A and a β of 300, because of the transistor configuration in Ngspice:

I_Base = I_Collector / Beta = 0.02/300 = 6.67 × 10-5 A

Using a rule of thumb: I_R1 = 10 × I_Base:

I_R1 = 10 × 6.67 × 10-5 A = 6.67 × 10-4 A or I_R1 = 0.667 mA

Using this current and knowing V_BE approx 0.6:

V_R2 = V_BE + R_E × I_E V_R2 = V_BE + R_E × (I_C + I_B) I_C is approx I_C + I_B, therefore

V_R2 = V_BE + R_E × I_C

R_2 × I_R2 = V_BE + R_E × I_C or R_2 × (I_R1 - I_B) = V_BE + R_E × I_C

Since I_R1 - I_B approx I_R1:

R_2 × I_R1 = V_BE + R_E × I_C

R_2 = (V_BE + R_E × I_C) / I_R1

R_2 = (0.6 + 56.25 × 0.02) / (6.67 × 10-4) = 2586.20 Ω

R1 = V_R1 / I_R1 = (9 - 2586.20 × 6.67 × 10-4) / (6.67 × 10-4) = 10907.053 Ω

Here is the circuit: enter image description here

And here is the simulated output: enter image description here

My question is why the voltage is not amplified by the circuit and just the current is.

I made some calculations in a Excel and it gave me a power gain of around 3.3. I was wondering if it is possible to make it higher or is this type of circuit not designed for that use? If so, which type of amplifier should I use?

  • Circuit number 2 - Try for max. current gain resistor selection

enter image description here

enter image description here

enter image description here

For this circuit I tried to select the resistors in order to set the Q-Point as high as possible.

I tried to take into account that while the current base increases the same difference in base current relates to a higher difference in collector current. Ergo: as I_B increases -> delta I_C / delta I_B increases

So I used a I_C when V_EC = 0 of 0.06 A and a I_C for Q-point of 0.04 A.

Nonetheless the voltage gain seemed the same.

I built both circuits with the closest real resistors possible and even a second stage, with the same resistor values as the first, and it does not seem a useable amplifier for the microphone; the signal increases but not as much as I expected at the beginning of this journey.

I would like to ask some general questions about amplifiers:

  1. Which circuit design should I use in order to amplify the voltage signal as well as the current signal?
  2. Which type of amplifiers do you recommend for low-noise output?
  3. How it is possible to avoid the feedback effect of a loudspeaker next to a microphone? (I used the circuit using a guitar amp and at 1/3 max gain, where the microphone was next to it, it started piping with no end of the peeeep until it was switched off.)
  4. It is normal to have a power gain of 3 for a β of 300?
  5. Is there something wrong in my calculations? I just started with it so there will be a ton of misunderstandings by my part, glad to be corrected.
  6. I used a V_BE of 0.6 V because the transistor is made of silicon. In Ngspice this value cannot be found anywhere. The nearest value I was able to find for this model was the base-emitter junction built in potential, Vje = 0.75 V or the substrate junction built in potential, Vjs = 0.75 V. Which one of this values would refer to the V_BE of the transistor? Or is it calculated in Ngspice with other parameters?
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    \$\begingroup\$ You can't arbitrarily amplify voltage and current as one of them, together with the load impedance, will determine the other. The real challenge is that your load is 35 Ohms and you're relying on a 150 Ohm resistor to drive it. \$\endgroup\$
    – Finbarr
    Jan 26, 2023 at 19:46
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    \$\begingroup\$ I'm not sure if I'm seeing ghosts or if others just aren't noticing. But I think that's an electret, not a DC-biased condenser, Man789. Which means it needs a managed input design. And if you intend to amplify a signal you need to understand both the input source as well as the output load. And more -- such as the likely SPL range you want to handle. It's physics first, then transducer to make electrical signals from physics, then nearly trivial electronics, then another transducer back to physics, then your ear and brain after that. \$\endgroup\$ Jan 27, 2023 at 2:01
  • \$\begingroup\$ You are right periblepsis, it is in fact an electrec microphone, I saw it called electret condenser microphone online, so I called it condenser microphone. I did not know it can also be used for "true condenser mics". I have the electret microphone in series with a resistance in order to achieve around 5V between plus and ground (of the mic). I have draw it in the simulation as a AC voltage source because I do not know how to represent it in another way. What do you recommend to read in order to understand about SPL? I guess it means Sound Pressure Level (range). \$\endgroup\$
    – Man789
    Jan 27, 2023 at 9:58

6 Answers 6

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There are a number of problems with your approach that need to be addressed before calculating anything.

The problem with this popular so-called "voltage amplifier" is that it's actually a transconductance amplifier (because of the emitter degeneration resistor, or series-series feedback) whose output current is transformed into a voltage through the resistance at the collector.

The gain will be given by: $$ A_v = -\frac{R_{collector}}{R_{emitter}} $$

However, one can get away with it ONLY IF the load resistance is a lot of bigger than \$R_{collector}\$, because, that way, the voltage gain is defined only by the collector resistor.

But you have the opposite! So this is hopeless. Your load is defining your voltage gain.

On top of that, it's not reasonable to voltage and power amplify your input signal in one single stage. This is normally done with 2 or 3 stages.

As a simple way forward, I would suggest you simply use an op-amp with a buffer within the loop, perhaps a non-inverting one. You could use the LT1010 that has +/-150mA outputs current, which can produce a little more than a 5V max with a 35ohm load.

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  • \$\begingroup\$ Why do you use the term "drain" instead of "collector". There is no FET in that circuit. That's confusing, especially for an hobbyist and in particular because it is unclear the relationship with the OP's circuit, in which there are R2 and R3 (or R4 and R5), instead. \$\endgroup\$ Jan 27, 2023 at 12:54
  • \$\begingroup\$ @LorenzoDonatisupportUkraine corrected the terminology. I didnt use his resistor names because they're different in both schematics. I guess he knows what is the collector and what is the emitter. \$\endgroup\$
    – Designalog
    Jan 27, 2023 at 13:05
  • \$\begingroup\$ Again, you used "drain", not "collector". That was the main point of the confusion, IMO. But I see that you corrected that, so it's OK. \$\endgroup\$ Jan 27, 2023 at 13:52
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    \$\begingroup\$ Gracias Ermesto! Yeah I could understand what you meant by drain. Thanks for pointing about the resistance difference, I have read that the output impedance should be 1/10 th of the input impedance of the load. That is why I wasn't getting any voltage gain. Sadly I will need to use a different circuit or different transistors, because using a R_C lower than 1/10 the load resistance seems to not work under the simulation. \$\endgroup\$
    – Man789
    Jan 29, 2023 at 16:25
  • \$\begingroup\$ @Man789 I'm thinking about making some YouTube video series about how to properly design a feedback transistor amplifier that can comply with different specs. I see most people in this forum tripping over the "common-emitter amplifier" again and again. \$\endgroup\$
    – Designalog
    Jan 30, 2023 at 10:32
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You're not going to be able to drive a low impedance load like a speaker very well with a 2N3904 based common emitter amplifier. You need an amplifier with an output impedance significantly lower than the load impedance.

You should look into using something like a complementary symmetry push-pull amplifier. This uses an NPN and a PNP transistor as emitter followers which present a much lower output impedance. It won't give you any voltage gain though so it needs to be driven with a voltage amplifier, this is where you might use a common emitter amplifier.

There is plenty of information on building these online, you should be able to find everything from very simple hobbyist amplifiers to high-end audiophile amps using complementary symmetry output stages.

Here's an example of a basic complementary symmetry amplifier. complementary symmetry amplifier complementary symmetry amplifier waveforms

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  • \$\begingroup\$ Thanks. I already made some simulations of a class AB amplifier. Is it what you refer as "complementary symmetry push-pull amplifier", isn't it? Nonetheless I have some understanding problems about these and some troubles biasing the two transistors, I was using 3904 as NPN and 3906 as PNP. But couldn't achieve a correct base current for both at the same time. As I understood do you recommend me to use a driver stage amplifier? Furthermore I have a doubt about class AB amplifiers: \$\endgroup\$
    – Man789
    Jan 27, 2023 at 10:09
  • \$\begingroup\$ I know it is needed to set the Q-Point in order to the transistor to conduct just with positive or negative voltages, not both (with a little of shared conduction zone if needed). The question would be which base current select for the amplifier to work in this region. Taking as example the characteristics curve plot in the first message for the 3904 would it be at least 50 uA or does it need to be lower? I mean there is a line on the bottom which should be 1uA (without the current value) and because of it I am not sure which base current use in order to bias the transistor. \$\endgroup\$
    – Man789
    Jan 27, 2023 at 10:13
  • \$\begingroup\$ @Man789 Complementary symmetry (CS) is a circuit topology using transistors of opposite polarity (NPN/PNP, NFET/PFET, NMOS/PMOS). AB is a class of operation based on the bias point. A CS amplifier can be biased as class AB, but it can also be biased as class A or class B. Which bias is used depends on how much you're willing to sacrifice efficiency for low distortion. Class A will waste at least half of your power as heat but have low distortion, class B will have crossover distortion but be much more efficient, class AB tries to find a happy medium between the two. – \$\endgroup\$
    – GodJihyo
    Jan 27, 2023 at 15:19
  • \$\begingroup\$ How can a complementary symmetry circuit topology be biased as a class A amplifier? Would it have two output maybe? Thanks for the correction! And for your answer to my question as well! \$\endgroup\$
    – Man789
    Jan 29, 2023 at 16:33
  • \$\begingroup\$ @Man789 You just increase the bias so both output transistors are running class A. You lose the efficiency of class B or A/B, but get low distortion along with the capability of driving low impedance loads that you wouldn't have with a simple common emitter amplifier. You can increase the bias in this circuit by adding another diode or two, or better yet use a Vbe multiplier (en.wikipedia.org/wiki/Rubber_diode). Class A generates a lot of heat so you'd need to go with higher power transistors. For what you're doing B or A/B would be better anyway. \$\endgroup\$
    – GodJihyo
    Jan 29, 2023 at 18:15
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You could try something like this ...
But be aware that it can't be used with "batteries" ... ("Big" average power supply).

enter image description here

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  • \$\begingroup\$ Thanks! I will try it right now. Could you please tell me what's on the vertical axis of the plot? And what it cannot be used with batteries? Maybe because of the current needed? Furthermore if you could tell me the software you are using would be nice. Just curious. \$\endgroup\$
    – Man789
    Jan 27, 2023 at 9:40
  • \$\begingroup\$ The blue curve is the amplitude signal generator( no harmonics). Red curves are the output voltage amplitude of the fundamental and the harmonics (lower than fundamental "obviously"). Current needed is > 100 mA ... Software used is FREE microcap v12 spectrum-soft.com/download/download.shtm . \$\endgroup\$
    – Antonio51
    Jan 28, 2023 at 17:52
  • \$\begingroup\$ Thanks a lot! I will check the software as well as try the circuit with the transistor 2N2119A. \$\endgroup\$
    – Man789
    Jan 29, 2023 at 16:32
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Why is your schematic covered with meters instead of a simple list of voltages? Why do you test with an extremely low frequency of only 1Hz instead of audio? Why is your input level extremely low at only 100uV?

Here is my simulation of your circuit with a 10mV peak 1kHz input and mine with a 1500 ohms load: transistor

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  • \$\begingroup\$ Just started with Spice simulations. I used probes instead of nodes names because it is easier to see for me. I used 1 Hz instead of 1 kHz or 10 kHz because it is easier to see the plots values and it does not make any huge difference in the results. I used a 100 uV AC input signal because I do not own an oscilloscope yet and my multimeter goes as low as mV for AC signals and could not read any value going out from the mic circuit. Nonetheless connecting the mic to a computer the sound can be recorded but with a really low volume, so I guessed the 100uV value for the AC signal. \$\endgroup\$
    – Man789
    Jan 27, 2023 at 9:20
  • \$\begingroup\$ Thanks for the notes next to the simulated circuit, I will modify the circuit in order to see the output. About the load, I will use a 35 Ohms ear-speaker as load. So I will need to use a R_4 of maximum 3,5 Ohms? Or do I need a two stage amplifier for sure? \$\endgroup\$
    – Man789
    Jan 27, 2023 at 9:21
  • \$\begingroup\$ If R4 is only 3.5 ohms then the transistor and battery current will be a massive 1A and they will burn up. Get rid of the antique class-A single transistor idea and use class-AB two complementary transistors for an output and a third transistor feeding them for some voltage gain. The complementary transistors idle current will be only 5mA or 10mA, not 1A. \$\endgroup\$
    – Audioguru
    Jan 27, 2023 at 19:12
  • \$\begingroup\$ My goal is to have some different amplifiers in order to be able to listen to the differences between them. I am not saying I will not do a class AB amplifier, in fact I am currently working on 4 types of them, nonetheless I would like to build a class A amplifier just for the fun. I will try to check with other transistor as changing the collector resistance to a 0.5 ohms does not seem to work for the 3904. \$\endgroup\$
    – Man789
    Jan 29, 2023 at 16:29
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Regarding a microphone ("pre"-)amplifier, see Tony Stewart EE75' answer, too.

There is no single stage amplifier from electret microphone output to ("passive", no-amplifier) speaker in a single stage, BJT or otherwise.

  1. Feedback and pop suppression look almost manageable using digital signal processing.
  2. compute the (AC) power gain from base to (loaded) collector
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  • \$\begingroup\$ Thanks! I was already using this circuit for other simulations and trying to understand how to bias it correctly. The link seems an interesting read. I am using a guitar amplifier as "Load" as well. The sound recorded by the microphone sounds quite low. I tried the microphone alone, a one stage amplifier and a two stage amplifier. Nonetheless I guess I made it wrong with the two stages amplifier as I used the same resistor values for both stages. Now I guess I need to used different resistors values in order to assure a input impedance of 2nd stage 10 times of output impedance for the first. \$\endgroup\$
    – Man789
    Jan 27, 2023 at 9:35
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To the other schematic I added a bootstrap capacitor to increase the positive signal level and added an input resistor so that the negative feedback can reduce distortion. The biasing is changed a little. earphone amplifier

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