CLASS AB amplifier design

[vinay13]

can some one help me with the biasing of BD139 and BD140 to operate in classAB amplifier

 

[BradtheRad]

Screenshot showing the general idea:



Your incoming waveform must be at about the same amplitude you want to appear on the load. This topology provides current gain, not voltage gain.

Adjust the potentiometers so they just start to turn on the transistors. Too little and you get crossover distortion. Too much and you waste current when there is little or no signal.

 

[Ow@i$]

as said by Brath!! adjust base voltage below midpoint of IV curve to ensure minimum crossover distortion otherwise it will be B class amplifier!!!

 

[vinay13]

what u said is correct but i have used diodes for biasing the base of BD139 and BD140 to 0.7v and i want to get maximum current from the class AB around 1Amp and my input is 30Vp-p sine wave.but one more thing i noticed is tht as i reduce the input wave peak to peak voltage the current drawn increases and vice versa. one more thing is tht even whn there is no input signal the transistors still draw current which indicates tht the transistors arent biased into class AB configuration i suppose . plz i need help urgently.

 

[Ow@i$]

how you checked that it is drawing current with no input signal???
if it is drawing that means it is in A-Class region!!!first keep the input voltage fix according to what you require!!! then use potentiometer first to... and while changing bias point steadily study its behavior !!! and see where you are getting max current!!! don just fix it straight away!!!

 

[BradtheRad]

what u said is correct but i have used diodes for biasing the base of BD139 and BD140 to 0.7v and i want to get maximum current from the class AB around 1Amp and my input is 30Vp-p sine wave.but one more thing i noticed is tht as i reduce the input wave peak to peak voltage the current drawn increases and vice versa. one more thing is tht even whn there is no input signal the transistors still draw current which indicates tht the transistors arent biased into class AB configuration i suppose . plz i need help urgently.

It is typical to see a certain amount of wasted current when the amplifier is quiescent.

Screenshot of normal operation when bias is adjusted to deliver 1 amp to load (incoming waveform 15V amplitude):



Screenshot when input waveform is flat line, showing 93 mA wasted when idle:

 

[FvM]

but i have used diodes for biasing the base of BD139 and BD140 to 0.7v and i want to get maximum current from the class AB around 1Amp

How about showing your circuit?

 

[treez]

Yes, AB amp you supposed to get current flow even when no signal.

A nice thing abotu AB class amps is that your input voltage doesnt matter too much.
......that is , if you are using a rough mains transformer based power supply, then if the V(in) is a few volts too high, then it really deosnt matter and wont effect your sound, but will mean that it doesnt start clipping till the power goes higher.

Your big question is what size to make your heatsink for your transistors.
Because typical sound has low power, so you can get away with a small heatsink, -but is this risky?...what if someone plugs in a "funny" instead of a microphone?

 

[vinay13]

here u go thts my circuit
my circuit draws around 550mA of current and the load is a coil.
i want to draw maximum current and supply to the load .

 

[FvM]

my circuit draws around 550mA of current and the load is a coil.
i want to draw maximum current and supply to the load .

There's a high risk to exceed BD139/140 power rating in your circuit. I guess you'll want to use "stronger" transistors.

 

[BradtheRad]

here u go thts my circuit View attachment 88997
my circuit draws around 550mA of current and the load is a coil.
i want to draw maximum current and supply to the load .

The idea is to drive the transistors to their minimum 'On' resistance. That is how you will obtain maximum swing. To do this, you need to apply large signal current, as well as sufficiently large bias current. Hence it works best if your biasing arrangement contains the least amount of components.

Your circuit can work. It is typical of class AB designs which may be found on the internet. The diodes create an automatic voltage separation between the upper and lower bias wires. However if the diode has a higher threshold V than the bias, it starts to waste excessive quiescent current. The transistors are asked to dissipate excessive watts as heat.

So, after trying many simulations over time, I find the post #2 schematic allows you to fine-adjust values so as to waste the least current. This is important, because you are powering your amplifier from a high supply V. The voltage drops become substantial enough to create several watts of heat in your transistors.

 

[vinay13]

can someone help me with short circuit or over current protection circuitry for class AB amplifier...!!

- - - Updated - - -

The idea is to drive the transistors to their minimum 'On' resistance. That is how you will obtain maximum swing. To do this, you need to apply large signal current, as well as sufficiently large bias current. Hence it works best if your biasing arrangement contains the least amount of components.

Your circuit can work. It is typical of class AB designs which may be found on the internet. The diodes create an automatic voltage separation between the upper and lower bias wires. However if the diode has a higher threshold V than the bias, it starts to waste excessive quiescent current. The transistors are asked to dissipate excessive watts as heat.



So, after trying many simulations over time, I find the post #2 schematic allows you to fine-adjust values so as to waste the least current. This is important, because you are powering your amplifier from a high supply V. The voltage drops become substantial enough to create several watts of heat in your transistors.

I replaced BD139 and BD140 with BDW42G and BDW46G and now my class AB amplifier is working fine...!!! thanx for the help if u have anymore information on class AB u r welcomed to share it and also some designs on over current and short circuit protection circuitry for class AB amplifier...!!

 

[BradtheRad]

can someone help me with short circuit or over current protection circuitry for class AB amplifier...!!

I replaced BD139 and BD140 with BDW42G and BDW46G and now my class AB amplifier is working fine...!!! thanx for the help if u have anymore information on class AB u r welcomed to share it and also some designs on over current and short circuit protection circuitry for class AB amplifier...!!

Good going. Congratulations!

You particularly want to protect your speaker. A fuse inline with your speaker is a good idea.

Also a fuse on both supplies, rated for 1.5x the peak current which you anticipate at loud volume. (With both supplies fused, I suppose you will not need any fuse inline with the speaker.)

The fuse ought to be fast-blow, not slo-blow. The hazard is from either transistor going short-circuit. This would expose your speaker to 26 V, at 3 or more amperes. It would need a very high watt rating, to avoid having a ruined voice coil in your speaker.

If both transistors were to go short simultaneously, then your power supply would be ruined, except if you have a fuse on them.

 

[vinay13]

no my load is not a speaker its a coil having a primary winding and a secondary winding i want to put lot of current and peak to peak voltage in the coil so tht the induced voltage is high and thts the reason i want protection circuitry in case of short circuit and over current.here is the design the 2N3904 and 2N3906 along with 1Kohm resistor forms the protection circuitry can u help me with the calculations needed to be done in order to choose values for the protection circuitry.

 

[godfreyl]

can u help me with the calculations needed to be done in order to choose values for the protection circuitry.

The only thing you need to change is the value of the 1Ω emitter resistors at the output. The 2N3904 and 2N3906 transistors will switch on and limit the output current when their base-emitter voltage reaches about 0.7V.

So maximum output current = about 0.7V / Re. With Re = 1Ω, the output current should be limited to about +-0.7A

 

[vinay13]

The only thing you need to change is the value of the 1Ω emitter resistors at the output. The 2N3904 and 2N3906 transistors will switch on and limit the output current when their base-emitter voltage reaches about 0.7V.

So maximum output current = about 0.7V / Re. With Re = 1Ω, the output current should be limited to about +-0.7A

yes thts true but do i need to connect a current limiting resistor at the base of 2N3904 and 06????
and how do i test the protection circuit to see if its actually working the way it is supposed to ??

 

[godfreyl]

do i need to connect a current limiting resistor at the base of 2N3904 and 06????

It may not really be needed, but it is a good idea, just to be safe. 1K is fine.

and how do i test the protection circuit to see if its actually working the way it is supposed to ??

I would connect a low resistance load to the output, then slowly increase the signal amplitude, while watching the output with an oscilloscope, to see what voltage the output is limited to.

 

[vinay13]

fine i'll try doing tht and what if i directly short ma output with the ground??? will my circuit still be protected from overcurrent???or i shouldnt be doing tht?

 

[FvM]

fine i'll try doing tht and what if i directly short ma output with the ground??? will my circuit still be protected from overcurrent???

It depends...

It's safe, if you're transistors have sufficient heatsink, Rth < 4-5 K/W.