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Bias design, amplifier design, filtering etc.

transistors need a different bias voltage for the same idle current even transistors of the same type, a bit more than .6v for 2879's but its always wise to start low and adjust for the correct idle current,
how much the pair of transistors will draw from the bias supply at full output power largely depends on the dc current gain of the transistors and how much current the collectors are drawing,
collector current / hfe ( at that operating point )
will give you a clue as to how much current your bias supply needs to provide,
low beta transistors draw more current at the same amplifier current draw,
at idle the current draw from the bias supply is much lower,

your main concerns with a regulated ab circuit are that it gives the correct voltage for the idle current you desire in a harsh rf environment and that it has a high enough current reserve to stay there as drive is applied to the amplifier, some voltage sag is acceptable to the ears and unless you use active regulation you will always have some bias voltage sag/modulation,
another is that it tracks temperature to keep the transistor operating point within your desired limits,

is it worthwhile adding current limit?
maybe, having the bias voltage fold back at your maximum input or a little below may give a bit of protection if you overdrive the amp,

a single transistor amp is good to practice on;)
 
Since transistors are current controlled devices, and since the base-emitter voltage has quite a bit of variability between devices and at temperature, why don't you design a constant current regulator to control base current for the amplifier class - which I'm assuming is AB?

If you want to use volatge regulation, you would be much better off using an FET type device instead.
 
can you elaborate on that ken?,

we are biasing bipolar devices where base voltage needs to stay fairly constant under variable drive/load conditions, the circuit also needs to thermally track the final devices and the pass device in none active circuits to compensate for increases in forward conduction of the biased devices and pass element with increases in junction temperatures,

i have ci/cv bench supplys, in ci mode as you know they swing voltage to keep current constant,
base emitter current is far from constant in a driven class ab bipolar amp, as drive is increased base emitter junction conduction goes up with a corresponding increase in current drawn from the bias supply, if the bias supply cannot supply enough current then operating point moves backwards causing poor linearity,

if you used a ci bias supply it would start in class ab and swing back towards class c as drive is applied,
am i missing something?

i have thought of trying a mosfet pass transistor as the pass device in the basic regulator/pass setup to see if that eliminates positive drift with temperature of the pass device but the double tracking seems to work well with the simple regulator/pass setup.
 
bob85, with a constant current source, and a series diode to control/isolate the current flow between stages, the bias should stay AB and not go to C when a signal is applied.

With a constant voltage source, the currrent in the base will always change with temperature since the base-emitter voltage, and collecctor current changes with temperature.

What value of base current are you shooting for when biasing the amplifier as AB?
 
ken, i dont understand the idea when related to the amps we are biasing,

as an example,
say your 2879 has a hfe of 10 and at its rated output draws 10 amp, using the formular collector current divided by hfe we have 10/10 =1 so your bias circuit needs to be able to supply at least one amp to keep the operating point stable at all drive levels up to full power.

if the bias circuit cannot supply the current it folds back voltage as drive is increased causing a portion of your drive signal to be wasted just turning the transistor on and a less linear power transfere,

the same amplifier using none active regulated bias needs more drive for maximum power out but about the same drive as a regulated setup at low drive levels where bias current is much lower,

simple resisistive divider setups are even worse, they have enough current to bias the amp at idle but cannot keep it there under driven conditions,
one roughass way round it the brute force method is to overbias the transistors at idle aka texas star / messenger and use low impedance resistors in the divider network to minimise voltage sag due to load impedance swings,
it works you cant hear the switching distortion but its not linear,

i have seen cascode constant current sinks in pure class a audio amps but i dont get how replacing my constant voltage source with a constant current source will allow the transistors operating point to stay in class ab when the load the bias supply sees is changing by a large magnitude over the input drive range of the amplifier,

the way i see it is as the base emitter junctions ask for more current the constant current source will start to fold back voltage forcing the amp into a lower class of operation like a resistive divider setup only worse,,

if you dont see it like that please explain(y)
 
bob85, a constant current supply holds the current at a constant value while allowing the voltage to vary between its maximum and minimum operating space - there is no foldback.

A power supply can have current limiting which will fold back the voltage when the current draw is at or above the threshold power limit.

Can you post up a schematic of your bias and biased amplifier stage? Maybe I'm missing something...
 
ken i understand how a constant current supply operates,
if you used a ci bias supply with bipolar transistors you would set the biasing so that the transistors had the correct idle current,
been bipolar the current needed from the bias supply to give the correct idle current would depend on the hfe of the biased transistors, the problem with ci is that as you start driving the amplifier the base emitter junctions go down in impedance demanding more current from the bias supply, the constant current bias supply would fold back voltage to maintain a constant current pushing the amplifier towards class c,

a constant voltage supply keeps the voltage stable while supplying enough current to keep the transistors at the correct operating point under all drive conditions improving linearity,

i dont have a schematic of a cb amp handy, they are all pretty much alike in basic design at dc at least,
heres an amplifier that uses a very similar bias circuit and would have the same needs as far as bias supply is concerned,

http://www.communication-concepts.com/appnotes/AN762Sharp300.pdf

what do you think ken am i missing something?,
if i am then i woiuld like some explanation as to why i see the measurements i see when i bias or modify the bias circuit in an amplifier:mellow:
 
bob85, I am not sure what you are seeing or what type of circuit you have tested.

If a constant current is supplied to the base-emitter junction by the bias circuit, and there is a coupling capacitor to block the bias voltage and to allow the AC signal to be added, Kirchhoff's current law will be intact and the algebraic sum of the currents entering the base-emitter junction will be bias plus signal.

The load can change impedance values between 0 Ohms and some maximum ohm value which will be a function of the saturated supply voltage = Constant Current x Maximum Load Ohms.

The current doesn't change, so how can the class of operation change? I guess I am confused as well...

Maybe I should design you a circuit to test if you are willing?
 
how can it change bias class?,
from my measurements it looks like whenever the bias circuit cannot supply enough current to keep the bias voltage stable the base emitter junction uses a portion of the drive signal to turn the transistors on,

with regulated biasing you get a more linear input/output ratio than you do with an unregulated bias circuit that dips back to say .4v under drive conditions
i would say for best linearity you need a rock stable vcc and bias supply,

a constant current bias supply would imho act like a very poorly regulated bias supply,
i dont understand how a constant current bias supply can give the correct idle current and at the same time keep the same operating point on bipolar transistors with upwards of a 200fold change in collector current,

i would be willing to give your idea a test on some cheap audio transistors if the parts are readilly available.;)
 
bob85 said:
how can it change bias class?,
from my measurements it looks like whenever the bias circuit cannot supply enough current to keep the bias voltage stable the base emitter junction uses a portion of the drive signal to turn the transistors on,

with regulated biasing you get a more linear input/output ratio than you do with an unregulated bias circuit that dips back to say .4v under drive conditions
i would say for best linearity you need a rock stable vcc and bias supply,

a constant current bias supply would imho act like a very poorly regulated bias supply,
i dont understand how a constant current bias supply can give the correct idle current and at the same time keep the same operating point on bipolar transistors with upwards of a 200fold change in collector current,

i would be willing to give your idea a test on some cheap audio transistors if the parts are readilly available.;)
Click to expand...

I can design a simple voltage controlled current source for you to test. You can also download PSPICE here: http://www.eng.auburn.edu/~troppel/91pspstu.exe and trhe tutorial here: www.ece.utah.edu/~harrison/ece3110/PSpice92Tutorial2.pdf if you want to simulate it before your build it.
 

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