Heavy Amplifier Circuit Oscillation Happened

In summary, when I increased Vce of VBE multiplier just a tiny bit, then the current through both upper and lower output power quadrants suddenly increased to 2A or more and stayed there or it was swinging like there was some kind of heavy oscillation within amplifier! As soon as I added AC coupling capacitor (switch position A), the darn thing worked as expected when Vce of VBE multiplier was changed!
  • #1
Stonestreecty
20
3
TL;DR Summary
Heavy oscillation occurred within my amplifier circuit when there was no AC coupling capacitor between voltage amplification stage (MC1458) and output power stage, I don't know why.
Hi, all
I am experimenting with audio amplifiers for a while now. I learned how to do an operational amplifier with discrete components and understand all its section and sub-circuits, recently. Then I decided to just skip the hard part and use operational amplifier and output power stage for an audio amplifier to work as desired, instead of troubling with discrete components only. The goal from my earliest experimentation was to create around 100 W audio amplifier but the things went wrong as soon as applying DC to whole amplifier (no AC for now).

I have built this circuit few minutes ago on breadboard:
amplifier circuit.jpg

When I checked only output power stage with VBE multiplier (without MC1458 op-amp) everything worked just fine. But when I connected op-amp to it directly (no AC coupling - switch position B), as I increased Vce of VBE multiplier just a tiny bit, then the current through both upper and lower output power quadrants suddenly increased to 2A or more and stayed there or it was swinging like there was some kind of heavy oscillation within amplifier! As soon as I added AC coupling capacitor (switch position A), the darn thing worked as expected when Vce of VBE multiplier was changed! As it was increased with potentiometer P2 slightly also the quiescent current through power stage increased slightly - as expected!

Why did such heavy oscillation occurred within my amplifier circuit when there was no AC coupling capacitor between voltage amplification stage (MC1458) and output power stage (upper and lower pair with paralleled power transistors)?

By the way, C3, C4, C5 and C6 represent AC bypass to avoid distortion when AC is applied to such circuit. At the collector and/or emitters of power transistors Q4, Q5, Q6 and Q7 there are no low resistance resistors for improving thermal stability of those because I have no low resistance (lower than 1Ohm) high power resistor. Also, I have replaced MC1458 with another one (same part), but the results were similar (talking about example with no AC coupling capacitor).
 
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  • #2
Did it sound like a series of pops?
What was the frequency of oscillation?
Were your power supplies stable?
I would look for an oscillator involving C1, where the phase shift is partly canceled by C2.
 
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Likes Tom.G
  • #3
It looks to me that without the coupling capacitor the system is entirely DC coupled. This means that there is no low frequency limitation, and so that very low frequency oscillation is possible perhaps as a result of common impedance coupling via the power supply or the connecting leads. It is better to restrict the LF (and HF) response of an amplifier to what is actually required.
 
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Likes Tom.G and berkeman
  • #4
Stonestreecty said:
I have built this circuit few minutes ago on breadboard:
That may explain the problem. Breadboards have high inductance, resistive connections and poor grounds. You will encounter many unexpected problems with circuits built on breadboards.

Another point. The op-amp has JFET inputs with very very low input current. There is little point in your Rbias of 47k, you can throw it out. Only high input current bi-polar op-amps needed that resistor. Op-amp offset voltage will swamp the input bias current error, and any offset will disappear in the feedback loop.
 

Related to Heavy Amplifier Circuit Oscillation Happened

1. What is a heavy amplifier circuit oscillation?

A heavy amplifier circuit oscillation is an unwanted and potentially damaging phenomenon that occurs when there is an unstable feedback loop in an amplifier circuit. This results in a continuous and uncontrolled output signal, which can cause distortion and even damage to the components of the circuit.

2. How does a heavy amplifier circuit oscillation happen?

A heavy amplifier circuit oscillation can happen due to various reasons, such as incorrect component values, poor grounding, or improper layout of the circuit. It can also occur if there is a defect in one of the components or if the circuit is being overdriven.

3. What are the signs of a heavy amplifier circuit oscillation?

The most common signs of a heavy amplifier circuit oscillation are a distorted output signal, excessive heat generation, and abnormal sounds coming from the circuit. In some cases, the circuit may also shut down or fail completely.

4. How can I prevent a heavy amplifier circuit oscillation?

To prevent a heavy amplifier circuit oscillation, it is essential to carefully design and test the circuit before implementation. This includes using appropriate component values, proper grounding techniques, and a well-designed layout. It is also important to avoid overdriving the circuit and to regularly check for any defects or malfunctions.

5. How can I fix a heavy amplifier circuit oscillation?

If a heavy amplifier circuit oscillation occurs, the first step is to identify the root cause of the problem. This may involve checking for incorrect component values, poor grounding, or any other potential issues. Once the cause is identified, appropriate measures can be taken to fix the circuit, such as replacing faulty components or adjusting the circuit layout.

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