Audio: Discrete Power Amplifer

8/11 2012

several audio power amplifier for receiver.   

Revised SM0VPO audio amplifier

This small audio amplifier is a revised version to SM0VPO's <<Beginners 250mW AF Amp>> . highly recommended this audio amplifier for your   receiver.

    * 250mW to 8R (5.6Vpp)  @7.2V supply ,  500mW  to 8R @9V supply  

    * very stalbe DC bias  cause of  DC negative feedback

    * Supply from 6V to 15V, widely usable without change anything

    * Gain determined by R4/R9,  eliminate the need of pre-amplifier for detector.

NOTE: C7 is wrong Polarity， should connect + to the speaker

Here is my version improvement/changes:

1)  add C2,  phase compensation, eliminate the high frequncy noise.

2)  R6 reduced to 2.4k, cause 8050 current gain is insufficient, need more drive. but don't keep reduce it. I burned the T3/T4 when R6=1.2k, the NFB can not handle the Thermal stability problem.

3) R9 is added to reduce the quiescent current.  510R is the point where crossover distortion just shown on oscilloscope @7.2V supply. 

And here is quick project pattern:

NOTE:

  keep the R5, R10  above 2R, smaller one will damage the Thermal stability.

this is so good to design a fly ground schema for this(@2014/6):

 

NOTE: if use plugin ear phone, make sure power up after plug the phone, or  VCC to GND current will burn your ear during  plug it in.


Revised SM0VPO audio amplifier V2


Modify bootstrap method. This way the earphone been isolated form power. It's more safety to experiment, I burn one earphone by accidentally short it to ground.

a new quick route for this amplifies:

The modified  EMRFD power amplifier

   

I reviesed <<EMRFD>> chapter one's discrete audio amplifier,  add bootstrap to the circuit make it a little bit louder.  the amzaing feature about this audio amplifer is widely supply voltage adaptability, from 5V to 18V the midpoint voltage almost fixed at 1/2 VCC and very stable cause the DC negative feedback arrange.

   * power supply: 5V~18V

   * quiescent current 3mA@7.2V supply

   * input: 200mVpp, output 3Vpp to 8R load , 140mW@7.2V supply

The output swing limited by 2 factor:

1) the raise swing can reach almost to the VCC because of the bootstrap.

2) the down swing limited by T2's biases voltage, the T2's collector can not low than 1.4V cause it's base fixed at 1.24V. plus T4's Vbe the lowest down swing voltage about 1.4V+0.6V=2V.

3) R10, R5 further limited the swing at 7.2V supply max swing output about 3Vpp to 8R load, about 150mW.

It's is enough louder after boxing, and THD is not good but acceptable for receiver.

I revised common small audio power amplifier for given much power but with much quiescent current.

*the supply voltage is optimized for 2 Li-Po battery, 7.2V to 8.4V.

*work point is marked

*output power to a 4R speaker about 4Vp-p, 500mW

The PCB is optimized for size, it's very small, 1.5mm x 4.5mm. easy to put to a hand phone size box, i.e.  the popular minty box.

audio power

here is a simple power amplifier for your toy, almost same as above, but more powerful.

DC NFB mini power amplifier

@7/18 2012

NOTE: the 8R should be  > 64R, otherwise the circuit is does not work.  the voltage gain around 150, use 4V supply max output swing is around 1Vpp.

Note from talkingelectronics:  47R is the key part, if a large on used, the 100uF can not be efficiency  discharged lead to little power to delivery.  another formal way to descriptive  this is:  follower can't delivery the current more than quiescent current.


bias: the 100k voltage drop will be more than 0.6v, you can't just omit it. this push the BC547 collector to a little high voltage. 

here is the simulation result.