THD of single BJT stage

[3n2323]

to explor BJT's, i built this single BJT amplifier stage on breadboard.


it seems to be working fine.


no equipments to explore further with, i used LTspice for the THD figure, it is a whopping 3.996%.


is this result normal?
how do commerial systems get their 0.1% or 0.01% figures?
is that because of negative feedback?
just for the purpose of exploring, how negative feedback can be added to the above circuit, so to knock down the THD figure?

thank you all for helping!

 

[w2aew]

Yep - this is typical.  The distortion will get worse as you increase the signal level.  Using a little feedback in the form of emitter degeneration will reduce the gain as well as the harmonic distortion. 

Just like most analog circuits - it's fairly easy to make a simple amplifier, it's difficult to make a really good amplifier.

[jimmc]

...just for the purpose of exploring, how negative feedback can be added to the above circuit, so to knock down the THD figure?...

Add a resistor in the emitter lead.
or
Connect R3 from base to collector (reduce its value) and add a resistor in series with C1.
or
Do both.

The first raises the input and output impedances, the second reduces them.

Adding a second stage increases the gain before feedback is applied and can give impressive results eg http://www.douglas-self.com/ampins/discrete/2Q-VEM/2Q-VEM.htm

Jim

[3n2323]

Yep - this is typical.  The distortion will get worse as you increase the signal level.  Using a little feedback in the form of emitter degeneration will reduce the gain as well as the harmonic distortion. 

added a 1K emitter resistor, THD dropped to 1% something, very effective, thank you!
output voltage dropped a lot as well, from 1V to 240mV, looks like it's a balancing game 

Just like most analog circuits - it's fairly easy to make a simple amplifier, it's difficult to make a really good amplifier.

is a good digital amplifier easier to make? i haven't touched digital yet.

[3n2323]

Adding a second stage increases the gain before feedback is applied and can give impressive results eg http://www.douglas-self.com/ampins/discrete/2Q-VEM/2Q-VEM.htm

thank you jim, really a good link!
the author is rigorously trying to make the circuit perform, just what i want to find out about how to.
i'll need to read it a lot more closely though.

[vk6zgo]

Reduce the value of the emitter resistor,add another resistor from the junction of R3 & C1 to ground.
Your base bias is now set by the voltage divider action of these two resistors.
You will have to mess around with the values of these to get the current through them much larger than ib.

This is called "Combination bias" & is the most common form of biasing in general use.

P.S. have a look at this site: http://www.zen22142.zen.co.uk/Design/bjtbias.htm

[ejeffrey]

is a good digital amplifier easier to make? i haven't touched digital yet.

There isn't really such thing as a digital amplifier...  People sometimes call class-D amplifiers 'digital' and indeed they are part digital but their analog side is actually far more subtle and interesting than a simple transistor amplifier.

Likewise, you can vary the signal level by multiplying in a DSP, but you still need the analog output amplifiers.  The advantage is that they can be fixed gain.

[Zero999]

A simple logic gate could be called a digital amplifier. Class D isn't really digital, it's PWM which is analogue.

[jimmc]

Some more good information here http://www.focalpress.com/uploadedfiles/Small.Signal.Audio.Design.Ch3.pdf particularly p 81 on.

Jim