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This 5-watt amplifier was designed to be an easy build for a novice and the parts used are common and used repeatedly through out. I spent a lot of time trying various combinations of components until I ended up with only 22 parts total, 8 resistors, 7 capacitors, 5 transistors, and 2 diodes.  And there are only 2 different resistance values used to keep things simple.

 

This is a stripped down negative feedback (NFB) amplifier using a dc coupled three-stage topology. This includes: the differential, voltage amplification, and current amplification stages and I am going to explain how it works.

 

The differential stage includes Q1, Q2, R3, and R4; here is a simplified overview of how the differential works. When the base of Q1 goes posistive it’s collector will follow if properly biased and the collector of q2 will go negative will respect to its neutral position equal in magnitude to q1’s collector. So they are out of phase with each other. Now if we apply the same signal to the base of the other transistor at the same time they will cancel each other and cause little if any change on the collectors. This is how you implement negative feedback, by taking a portion of the final output stage and feeding it back into the base of the transistor opposite the input signal transistor, you effectively cancel out a portion of the gain. Just like applying the same signal to both transistors at the same time cancels each other.

 

 

This Pspice simulator shows the signal relationship between the differential transistor pair. The red line indicates the input signal transistors collector and the green is the collector of the second transistor, so you can see how they are equal in magnitude but out of phase.

 

 

 

Differential stages are found in many circuits that need a means of comparing or detecting the differences between to signals and then give a result either a compared a subtracted or a summed.

 

Like most negative voltage feedback amplifiers the differential stage used here is detecting the difference between two levels, in this case; signal in and the signal provided by the R5, R6 voltage divider. The higher R6 to R5 ratio the higher the gain will be. This is because as the ratio goes up, less of the final output stage is being fed back into the differential amplifier and so less canceling is going on and while this sounds good more gain can also mean more noise. The gain of this amplifier is about 12 which is fine for my purposes but gains of 20 and higher are more common in commercial units using NFB.  

 

When one output of the differential is used but not both for providing the output it is called a single ended differential and this is what I have used here.

 

Also the differential stage helps with overall linearity and sound quality of the amplifier by sampling the output and compensating for the difference it finds between the feedback and the original input signal. It’s easy then to understand that a higher gain or less feed back in a NFB amplifier will cause noise levels to increase.

 

The voltage gain stage is just a simple bootstrapped class a amplifier with a compensation capacitor for high frequency stability.

 

This stage works by biasing q3 into class A mode, or always conducting, so that your final stages bias point is zero voltage or close to it. You can adjust this bias point by altering the resistance provided by R3 like with adjustable resistor. This amp works well though with just 1k resistor. The bootstrapped portion of this stage consists of C4, R7, and R8. How this works is when there is no

        

If your supply bypass capacitors are more then 10 inches from the amplifier circuit I recommend using two 47uf 25v electrolytic capacitors on the supply rails closest to the amplifier as well as the already installed .1uf ’s  

 

 

When there is no output the capacitor will be negatively charged with respect to ground but when the output goes high it creates an even greater difference across the capacitor so it’s charge now becomes the negative rail to ground charge of 20v plus how ever high the output swung, will say 2volts. So now the potential difference across the capacitor is 2 volts above ground and when the output starts to go negative the stored charge in the capacitor must go somewhere so it goes into the R7, R8 divider but remember the potential difference is higher then the negative rail to ground so the charge can only go through R7 and this assists in power the base of the output device. This is just a short cut method to get away with no Constant Current Source (discussed later).  And this cycle continues as long as the amplifier is being used.

 

The compensation capacitor is actually another form of negative feedback but because it’s a capacitor and not a resistor like in the feedback discussed earlier it will not function at direct current levels (DC) only alternating current levels (AC). Amplifiers have something called a gain bandwidth product, this is the point at which device's in the amplifier are no longer fast enough to provided proper negative global feedback (bad bad). The amplifier will start to oscillate and become useless possibly self-destruct when the negative feedback runs out, so we must stop the amplifier from getting there. An easy way of doing this is by applying a small capacitance across Q3’s collector to base junction. How this works is, a transistors collector will be out of phase with it’s base so just like discussed earlier if we apply a signal that is out of phase with an in phase signal they will cancel each other an equal magnitude of the phase difference magnitude.  This capacitor functions much like the capacitor in an ordinary speaker system crossover and gives roughly a 6db roll off above the target frequency.