3-stage BJT amplifier

[WeniTheElder]

I'm trying to build a 3-stage BJT amplifier to amplify the signal from an electret microphone and drive a 3W, 8Ω speaker. I'm kinda stuck and need some help.


My idea is to use the first two stages as common-base amplifiers, each with a gain of around 15.5. The mic outputs about 20 mV, and I calculated that the speaker needs around 4.9 V peak to deliver 3W, so I aimed for a total gain of about 240. The third stage is a common collector to help drive the speaker.


I chose capacitor values based on suggestions from ChatGPT, and for the resistors I assumed β = 100, VBE = 0.7V, and IC = 1 mA.


The problem is, when I simulate just the first two stages, I get a really distorted waveform, not a clean sine wave at all. I’m not sure if it’s because the gain is too high, or maybe the biasing is off.


Any advice would be appreciated — I just want to get a clean amplified signal before adding the last stage. Thanks!

 

[FvM]

Can't read much from oscilloscope waveforms without seeing offset/baseline and voltage range, except for seeing distortion. Also supply voltage is unknown.

What is "20 mV", Vrms, Vpeak or Vpp?

3W@8Ohm corresponds to 4,9 V rms respectively 7 V peak in my math.

 

[KlausST]

3W@8Ohm corresponds to 4,9 V rms respectively 7 V peak in my math.

true for pure sine waveform.

7Vp --> 14Vpp

***
The sound will be distorted.
Why not using a dedicated audio amplifier?

You say "3 stage", but I only see 2 stages.

Klaus

 

[WeniTheElder]

Can't read much from oscilloscope waveforms without seeing offset/baseline and voltage range, except for seeing distortion. Also supply voltage is unknown.

What is "20 mV", Vrms, Vpeak or Vpp?

3W@8Ohm corresponds to 4,9 V rms respectively 7 V peak in my math.

You're right — the 20 mV should be Vpeak, and I completely missed that the 4.9 V is the RMS value. Thanks for pointing that out!


I’m not sure how to properly display the offset and baseline on the Proteus oscilloscope, but here’s another screenshot — though it’s a bit messy with too many cursors showing.

 

[WeniTheElder]

true for pure sine waveform.

7Vp --> 14Vpp

***
The sound will be distorted.
Why not using a dedicated audio amplifier?

You say "3 stage", but I only see 2 stages.

Klaus

This is a mini project for one of my school. I was asked to build a 3-stage BJT amplifier. Right now, I’m just testing the first two common-base stages before adding the third stage, which will be a common-emitter.

 

[FvM]

adding the third stage, which will be a common-emitter.

Doesn't sound reasonable. Need at least class AB complementary output stage to drive speaker.

Bias point of preamplifier stages is obviously incorrect and must be corrected. First step, check present bias point.

 

[LvW]

This is a mini project for one of my school. I was asked to build a 3-stage BJT amplifier. Right now, I’m just testing the first two common-base stages before adding the third stage, which will be a common-emitter.

Common base? Are you sure?

 

[WeniTheElder]

Common base? Are you sure?

I was a bit high when I wrote that thread — the first two stages are common emitter, and the last one is a common collector.

 

[WeniTheElder]

Doesn't sound reasonable. Need at least class AB complementary output stage to drive speaker.

Bias point of preamplifier stages is obviously incorrect and must be corrected. First step, check present bias point.

sorry, the first two stages are common emitter and the last one is common collector.
sorry for the confusion

 

[kd502]

Component selection is unreasonable. Such a low effort attitude.

With proper collector resistor you will get over 40dB in one stage, and very poor linearity. You don't have any means to control this gain. Additional resistor in series with emitter capacitor is used.

 

[WeniTheElder]

Component selection is unreasonable. Such a low effort attitude.
View attachment 198994
With proper collector resistor you will get over 40dB in one stage, and very poor linearity. You don't have any means to control this gain. Additional resistor in series with emitter capacitor is used.

I’m here to learn. This isn’t “low effort” — it’s literally the best I know right now, and that’s why I’m asking questions.

 

[D.A.(Tony)Stewart]

Av = 240 voltage gain
Ay = 1.9k/8 Ohms = 240 conductance gain is related to current gain.
Each stage of a transistor choice must contribute to either voltage or current gain or both with compromise to both and linearity .

Is it even possible to achieve this using maximal power transfer theorem , MPT? This demands matched impedances for each stage. Understand? No distortion limits were given.

I think this is a learning exercise that challenges your ability to learn with all the history and tools to access that are at your finger tips. (Hint)


Define your task this way with specs, and do not limit yourself to a common config first and consider transformers like the early transistor radios in Japan.

You will succeed to learn more by trial and failure with specs and results and this demands more understanding from MPT and how to learn with web tools

--- Updated Yesterday at 10:43 AM ---

Let me show you my results

Ref: Grok3

Impedance Summary​

Stage	Input Impedance	Output Impedance	Load Impedance
Mic	-	5 kΩ	~5 kΩ (preamp)
Preamp (Q1)	~5 kΩ	~2 kΩ	~4.7 kΩ (driver)
Driver (Q2)	~4.7 kΩ	~500 Ω	~1 kΩ (power)
Power Amp (Q3)	~1 kΩ	~100 Ω (pre-T1)	8 Ω (via transformer)
Speaker	8 Ω	-	-

Maximum Power Transfer​

• Mic to Preamp: Input impedance (~5 kΩ) matches mic’s 5 kΩ for maximum power transfer.
• Preamp to Driver: Bridging (4.7 kΩ input vs. 2 kΩ output) prioritizes voltage transfer due to transistor constraints.
• Driver to Power: Bridging (1 kΩ input vs. 500 Ω output) ensures sufficient drive.
• Power to Speaker: Transformer matches Q3’s high output impedance (~100 Ω) to 8Ω, enabling 3W delivery.

Calculations​

• Total Gain: ~4900 (74 dB) to go from ~1 mV to 4.9 V_RMS.
  • Preamp: ~50 (34 dB, set by R4/R5).
  • Driver: ~10 (20 dB, set by R8/R9).
  • Power: ~10 (20 dB, including transformer).
• Power Output: P=VRMS2/8=4.92/8=3 WP=VRMS2/8=4.92/8=3W.
• Transformer: Impedance ratio Zprimary/Zsecondary=(Nprimary/Nsecondary)2Zprimary/Zsecondary=(Nprimary/Nsecondary)2. For 100:8 Ω, turns ratio ~3.5:1, stepping down voltage and matching impedance.

Practical Notes​

• Efficiency: Class-A is inefficient (~25%), drawing ~1–2A at 12V. A 12V, 2A supply is recommended.
• Transformer: A small audio output transformer (e.g., 100:8 Ω) is critical for impedance matching. Source from electronics suppliers (e.g., Mouser, DigiKey).
• Mic Bias: Use a 5V regulator or voltage divider for the electret mic’s 4.7 kΩ bias resistor.
• Tuning: Adjust R4, R8 for gain; test bias points (Q1, Q2 base ~0.7V above emitter).
• Distortion: Class-A may distort at high volumes; keep input signal <10 mV or add feedback (requires more components).
• Heat: TIP31C will dissipate ~5–10W; use a small heatsink.

Limitations​

• Three Transistors: Limits gain and efficiency. A Class-AB push-pull (two transistors in the power stage) would be better but exceeds the limit.
• Gain Distribution: Achieving 74 dB with three stages is tough; the transformer helps by boosting effective gain.
• Noise: Minimal filtering may cause noise; add capacitors.”

The keywords and specs are critical. To a better answer

​

 

[danadakk]

For starters electrets are high Z, but CE stages input Z low, so using hybrid pi model easy to compute Rpi :

Small Signal Analysis (Hybrid π model) for the Emitter Follower (CC) Circuit - CS Posts

Small signal analysis for the Emitter Follower Circuit.

csposts.com

Note table below, Zin fro CE a tad low, more often its somewhere around a few K in audio region.

Easy method to interface electret :

https://www.ti.com/lit/ug/tidu765/tidu765.pdf

 

[kd502]

I’m here to learn. This isn’t “low effort” — it’s literally the best I know right now, and that’s why I’m asking questions.

DC voltage at collector should be around half of the supply voltage. Capacitor in emitter circuit should have lower impedance at lowest frequency than resistor.
Try to make one stage work right.

It is difficult to make 3W power stage with one transistor.
Lowest part count don't make circuit simple. Lowest part count make circuit operate at its limits. Circuits operating at its limits are difficult to design. Such a circuit doesn't even work before tuning. Special components are required like transformer.

@D.A.(Tony)Stewart​

Low frequency circuits can/should be designed without using maximal power transfer theorem. Just add more transistors.
Two properly designed voltage amplifier stages, can make ~60dB, and adequate input/output impedances. But one transistor for common collector output stage is not enough. Transformers for output stage are expensive, and not easy to find, transistors are cheap.

@@danadakk
With emitter degeneration input impedance can be higher. Degeneration is recommanded to lower the distortion.

 

[danadakk]

Emitter de-gen will not displace the R2 4K bias R.....guess one could raise them as well
if transistor beta high enough.

Tradeoffs, always tradeoffs.....

 

[BigBoss]

You can not get 3W@8 Ohms with this configuration. You should place few stages in common emitter configuration then a Power Stage must be placed to drive 8 Ohms.