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help me plz,how 25w mosfet audio amplifier works?

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beheshtaein

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bd140 orcad

hi dear new friends,
i've just found a (nearly)famous circuit diagram:25w mosfet audio amplifier

but i don't know how it works,what is the role of every part in the circuit?
and how the resistors and capacitors have been calculated?

is there anybody who can help me??please i really need to know!

this is the circuit:
Amp25.GIF


Parts:

R1,R4_________47K 1/4W Resistors
R2____________4K7 1/4W Resistor
R3____________1K5 1/4W Resistor
R5__________390R 1/4W Resistor
R6__________470R 1/4W Resistor
R7___________33K 1/4W Resistor
R8__________150K 1/4W Resistor
R9___________15K 1/4W Resistor
R10__________27R 1/4W Resistor
R11_________500R 1/2W Trimmer Cermet
R12,R13,R16__10R 1/4W Resistors
R14,R15_____220R 1/4W Resistors
R17___________8R2 2W Resistor
R18____________R22 4W Resistor (wirewound)

C1___________470nF 63V Polyester Capacitor
C2___________330pF 63V Polystyrene Capacitor
C3,C5________470µF 63V Electrolytic Capacitors
C4,C6,C8,C11_100nF 63V Polyester Capacitors
C7___________100µF 25V Electrolytic Capacitor
C9____________10pF 63V Polystyrene Capacitor
C10____________1µF 63V Polyester Capacitor

Q1-Q5______BC560C 45V 100mA Low noise High gain PNP Transistors
Q6_________BD140 80V 1.5A PNP Transistor
Q7_________BD139 80V 1.5A NPN Transistor
Q8_________IRF530 100V 14A N-Channel Hexfet Transistor
Q9_________IRF9530 100V 12A P-Channel Hexfet Transistor

description:
Can be directly connected to CD players, tuners and tape recorders. Simply add a 10K Log potentiometer (dual gang for stereo) and a switch to cope with the various sources you need.
Q6 & Q7 must have a small U-shaped heatsink.
Q8 & Q9 must be mounted on heatsink.
Adjust R11 to set quiescent current at 100mA (best measured with an Avo-meter connected in series to Q8 Drain) with no input signal.
A correct grounding is very important to eliminate hum and ground loops. Connect to the same point the ground sides of R1, R4, R9, C3 to C8. Connect C11 to output ground. Then connect separately the input and output grounds to power supply ground.


Added after 19 minutes:

plzzzzzzz
it's urgent!:|
 

trapoe

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implementation of 25w mosfet amplifier

It's very complicated to write how every part works. It could take several pages of a good book, if you have a quite good background in analog circuit basics.
In this circuit you will find find RC filter (R3 with C3//C4), quite easy to explain, current generator (Q3 Q4 R4 R5), not so easy, differential stage (Q1 Q2).
And when you know how works all these parts of circuit you have analyse the behaviour of the whole circuit.
This is really a big work to do in a forum.
May be you can build it and if it works you can start to analyse the circuit.
 

beheshtaein

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best quiescent current bd140

thanks dear trapoe,
yes,you're right...
but some questions:
are these statements true?
- Mosfets are working as a push-pull AB class amplifier?
- R7 is making a feedback path to correct bandwidth and output resistance?
- R17,R18 make a voltage devider to amplify the output?

thx again...

Added after 3 hours 11 minutes:

i've simulated the circuit in orcad, but it didn't work correctly...
in the output (i put a sin generator with different amplitudes in the input) i saw only a dc voltage of about -3v!! no ac signal in the output?!!

i saw the currents in different part of the circuit, as you see in the picture below it seems that Q5 & Q6 are not active... i don't know what's the problem, Q6 is BD140 BJT transistor, i got it's pspice model from internet, maybe the model is incorrect?

or something else?

2vifbif.jpg
 

FvM

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trimmer cermet orcad

Q9 has drain and source terminal mixed up, unfortunately. Results in a nasty current of 243 A. Don't burn your fingers when touching the transistors on the computer screen!

B.t.w.: R17 and R18 are not a voltage divider.
 

trapoe

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Hi dear beheshtaein

Mosfets are working as a push-pull AB class amplifier?
Yes. As voltage across R11 increase it go "towards" class A, increasing dc current absorption with no signal. And (usually) diminishing distortion.
R7 is making a feedback path to correct bandwidth and output resistance?
Almost exact. R7, R6 and C7//C8 form the feedback path. Any negative feedback decrease gain (the closed loop gain is lower than open loop gain), increase bandwidth and decrease equivalent output resistance.
Note the two different feedback path: for DC (R7 only) and for AC (audio signal).
R17,R18 make a voltage devider to amplify the output?
Remember: a voltage divider cannot amplify.
R17 with C11 forms a frequency dependent impedance, with R18 act as "stabilization network" at high frequency.
When you make an amplifier with several active components it's quite easy that the output oscillate, generating a sine or other waveforms without ant input. R17, C11, R18 (with C9) try to avoid this risk.

Simulation results:
1) "mirror vertically" Q9. This way it probably act as a diode (it depends on the model) and this could be the reason for 243 A!
2) There are several strange behaviour in your circuit.
Q4 has Ic/Ib ≈ 1000 ! Q3 not much less. Too high gain.
Q6 has Ib ≈ Ie >> Ic this is strange. Perhaps the model don't works.

Try to check also Vbe of all bjt, usually 0,6 - 0,7 V.
Compare original and OrCAD circuit. (better don't hide value for bjt)
If you have doubt on models build a very simple circuit with one bjt and few resistors and try if it works.
Finally: your R20 is load resistance, use 8 Ω.
 

beheshtaein

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another simple questions:
pardon me because of this funny mistakes!
this time the problem is that:
-i don't get 25watt in the output in this conditions:
the input is:200mv sinewave 1kH
RL=8ohm but orcad shows 100uwatt in the output!!

-the circuit designer has mentioned that the current of R11 should be adjust to 100mA! but it's impossible to do that!

see the pictures:




Added after 10 minutes:

dear trapoe,
i've just seen your post! i wrote my post offline but when i sent it i understood that you've sent a reply!

so i'm sorry, i will follow your instructions and try to correct my mistakes!

thx all...

Added after 45 minutes:

hi again!
i think the problem of Q6 was solved by mirroring Q9...

but about Q3 & Q4: i replaced them with 2 new BC560C transistors from the library and the pspice model for them is :

********************************************************************************
.model BC560C PNP(Is=1.02f Xti=3 Eg=1.11 Vaf=34.62 Bf=401.6 Ise=38.26p
+ Ne=5.635 Ikf=74.73m Nk=.512 Xtb=1.5 Br=9.011 Isc=1.517f
+ Nc=1.831 Ikr=.1469 Rc=1.151 Cjc=9.81p Mjc=.332 Vjc=.4865 Fc=.5
+ Cje=30p Mje=.3333 Vje=.5 Tr=10n Tf=524p Itf=.9847 Xtf=17.71
+ Vtf=10)
* PHILIPS pid=bc559c case=TO92
* 91-08-02 dsq
********************************************************************************


the problem(gain is about 700 or 800!!!) isn't solved yet,what should i do?
it seems that gain should be about 400(since the model syas:Bf=401) is that right?

but here in this link you see that hfe of this transistor is between 380 & 800!
https://www.onsemi.com/PowerSolutions/search.do
 

trapoe

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Don't worry beheshtaein.
When you will add schematic I think it will better show voltages and currents, not power that's not so important.
R11: you're right, it's impossible to get 100 mA in R11. With R11 you can adjust the voltage between the two gates of Q8 and Q9 then the two Vgs then the drain current of both MOS. This is the current that must be 100 mA.
Before simulate with input signal you have to change R11 value to obtain drain current of 100 mA (approximately).

i think the problem of Q6 was solved by mirroring Q9...
Probaly mirroring Q9 change many voltages and currents in the circuit and now BD140 can works.

the problem(gain is about 700 or 800!!!) isn't solved yet,what should i do?
it seems that gain should be about 400(since the model syas:Bf=401) is that right?

but here in this link you see that hfe of this transistor is between 380 & 800!
I don't know so much about model but I agree. Following your link I saw small signal gain 450 - 900 but remember that hfe can vary a lot then Bf=401.6 is not so strange for a general model.

Don't worry to much about BC560 gain. Try first changing R11
 

FvM

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You apparently used the maximum value for R11, this causes an idle current of 12 A instead of 100 mA. With a real amplifier, it would be important to start R11 adjustment with minimum resistance value, otherwise the output transistors could be damaged.

I don't expect that the circuit is very sensitive to parameter variation, e. g. BJT current gain. It should work also with different parts in most positions.

As trapoe said, the node voltages would give more information. We can see from it, if the amplifier has a regular bias point or what may be probably wrong.
 

beheshtaein

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dear trapoe and FvM,
i learned a lot of new useful things here...
again thx for your attention and patient...

i made a short circuit in the input and then simulated,in R11=270 MOS drains current comes to 109mA(is that the same as what i should do to achieve the desired current?),the picture below is showing the point:


and now the sinewave is again added at the input:(this time i showed voltages)
does the amp have a regular bias point?

another question is that:can i calculate the harmonic distortion with orcad?

 

trapoe

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I used quite a lot OrCAD 9.2 for schematics and pcb but almost never for simulate circuits so I have to check if it's possible to have distorsion measured. By the way: which version of OrCAD you're using ?

The bias voltages seems reasonable.
Note that -29.92 mV at output is bias (DC) voltage. To see signal output you have to make graphics.
 

beheshtaein

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i use orcad9.2 too!

very happy to hear the circuit is working reasonably!

i'm waiting for your answer about how to calculate harmonic distortion with orcad...

thx
 

FvM

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The bias point seems O.K. so far. Distortions can be determined by activating fourier analysis in transient analysis.
 

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