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Projecting not very difficult amplifier...

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Junior Member level 3
Jun 29, 2002
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pls, could you advise me: how to chose a scheeme solving according to my needs, such as P, freq, garmonic.coeff, resistance of in-signal-generator & out-signal-keeper?

What functional scheeme should I use?

You are not specific enough. Try to specify your amplifier.

That`s it:
* U(in) = 0.18 V
* R(signal source) = 1200 Ohm
* P(load) = 21 W
* R(load) = 5 Ohm
* F(low) = 40 Hz
* F(high) = 18 kHz
* M(low) = 3.1 dB
* M(high) = 2.8 dB
* K(garm) = 0.5%
I wanna use double-tact (2-transistor out. stage) last cascade (`cos no need to use transformer),

How to choose transistors for this cascade (seems they should be complementary).

Just give me explanation. What is M(low) and M(high). Please.
Your problem is a simple audio amplifier as it seems at first look.

M is a coefficient of Magnitude(freq) charachteristic distortion on high and low freq...

Well. I sugest to search internet by Google for example for audio power amplifier schematics or project. I believe you will find suitable circuit to build. Output stage of amplifier must be full complementary simetry or quazi complementary simetry design. That means that output transistors must be complementary pair. Design is without output transformer directly driving load (loudspeaker). Your request for imput sensitivity can be acheieved by modification of feedback elements. Input impedance can be corrected simply.
If you need further assistance don't hasitate.

What`s about this?

What do you think if I use this scheme:
**broken link removed**
But I don`t wanna use BD139 `n BD140.
Really I need the last part of this scheeme (double-tact no-transformer cascade)???
Before last cascade I`d use two single-transistors cascades.
As soon as I wan`t use BD139,140 should I exclude two 0.22-Ohm resistors from tthe scheeme?


In this circuit the transistors BD139 and BD140 are in Darlington connection with output transistors. Without BD's circuit won't work.
In the case you don't want them you must exchange 2N3055 and MJ2955 and redesign driver stage.

The couples BD139/MJE2955 and BD140/MJE3055 form a couple of darlington transistor with a Vbe of 0,6 V instead of 1,2 V as usually is seen.
For this reason it is a must leave the two couples as drawn.
The two 0,22 Ohm resistors stabilyze the working point of the darlington and, with the two diodes, set the base working current.
If you cannot find the BD139/140 you can substitute with BD137/BD138 ( consider the working viltage of ± 35 Vdc ) or, for example, with the TIP 31C/TIP32C transistors.


If Excluding BD`s `n changing MJE`s to complementary 2N`s ,connect BJT-bases to the points between diodes `n 2.2kOhm-resistors, WILL it work? What do `u think?


You must not make those changes. Leaving 2k2 resistors will lead to limitation of positive half period. Actually 2k2(both) should be 120ohm.
This will cause driver to overheat (10W) and 120ohm res. should be 10W.
Such a change make no sens. Leave BD's in place!

Then. change your circuits with OP amp + Emitter Follower

Your application looks like low frequency operation circuit. I recommend OP amp + Emitter Followe Stage for easy implementation. Thanks. :idea:

1. Don`t wanna use OPamp. I know its much easier, but...
2. So `u want to say that there`s no way to reconfig scheeme for using only two BJT`s in the out-cascade?
Target is power not high voltage amplifying coeff...

By the way what`s for BD-140 base-emmit capacitor?

There is a way. Use darlington transistors instead BD 2N pair. Capacitors connected between base and collector lowers upper frequency limit and prevents from oscillation.

What do `u mean "prevents from oscillation"? Auto-generating?

No difference between using Darlington connected BJT`s or Darlington-transistors itself.

One thing I can`t understand is: why the scheeme wan`t be able to amplify if in the last cascade uses two power bJT`s (npn `n pnp) in the shared collector scheeme. Of course the freq wideness `ll fade a bit but...

Why does a capacitor stop oscillations?

Capacitors between B & C cause feedback that reduces gain at high frequency. This means that the amplifier can be designed so that by the time phase > 180 degrees, the gain is below 1 V/V, so it will not oscillate. Look up 'gain margin' and 'phase margin' in any text on electronic control for a better explanation.

wan't be able

Omitting darlington BD transistors means that driver stage has to deliver base currents for power end stage. At full power, in the positive half periode the base peak current of 2N3055 is approximately 40mA peak.
To achieve this two 2k2 resistors must be reduced. It means that the driver transistor which is in A class should draw significantly more steady state collector current which leads to increased power dissipation on driver transistor.

Dead n`t quite catch...The driver BJT is standing alone BD-139 in the middle-bottom of the scheeme???

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