No voltage gain in a common-emitter class A amplifier using a 3904 NPN transistor - Ngspice simulation

Question

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

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:

And here is the simulated output:

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

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?

Answer 1

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.

Answer 2

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.

Answer 3

You could try something like this ...
But be aware that it can't be used with "batteries" ... ("Big" average power supply).

Answer 4

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:

Answer 5

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.

3. Feedback and pop suppression look almost manageable using digital signal processing.
4. compute the (AC) power gain from base to (loaded) collector

Answer 6

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.