Transistor audio amplifer ?

Thread Starter

curry87

Joined May 30, 2010
101
Ive got some questions that need answers and since this forum is allaboutcircuits i thought its the place to post.


Im trying to figure out how to design a audio amplifier using just transistors and not opamps purely just for the educational value anyway on the input is a electret mic and on the output is a pair of 16ohm per channel earphones.

Which out of the two stages i posted is better for use in a audio amplifier ?

How many and what kind of transistor stages would i need ?

How is gain controlled in both of these circuits ?

Can someone post a transistor audio amplifier circuit that works without inductors and opamps ?
 

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Audioguru

Joined Dec 20, 2007
11,251
Your simple transistor circuits are preamps, not power amps. They would drive a power amp that drives a higher current into your headphones.

The first transistor uses a voltage divider at the base and the emitter resistor to provide a stable operating point.
The second transistor uses negative feedback from its bias resistor to provide a stable operating point.

The second transistor has a low input impedance caused by the negative feedback and its gain is affected by the source impedance.
 

MrChips

Joined Oct 2, 2009
19,159
The first circuit is the classic arrangement that gives best performance and temperature stability. The second circuit is a simplified bias arrangement that works well in a lot of audio applications. Try the second circuit first. Gains of 50 - 100 should be achievable.

(Do not connect the earphones directly to the outputs. Use a 10-100uF cap in series with the earphones).
(The electret mic needs DC bias to operate).
 

Jony130

Joined Feb 17, 2009
4,930

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Thread Starter

curry87

Joined May 30, 2010
101
Designing the amplifier is not a easy task.
Try read thous thread
http://forum.allaboutcircuits.com/showthread.php?p=170535#post170535
http://forum.allaboutcircuits.com/showthread.php?p=183239#post183239

And the voltage gain of the amplifiers you show is equal to
Av = Rc/re = 40*Ic*Rc
And this circuits you show are not so good as a audio amplifier.

And I can recommend you this circuit
Will this circuit you posted run off 9v if not what modifications are required ?

Is that pot 68k and 100k how it wired because a little confusing ?

How do i choose what input ac coupling cap value to use ?
 

Jony130

Joined Feb 17, 2009
4,930
Will this circuit you posted run off 9v if not what modifications are required ?
You need to change 150K resistor to 100K and replace two 180R resistors with one 220R and use 100K pot and set output voltage to 6V.

Is that pot 68k and 100k how it wired because a little confusing ?
The pot I use is 100K, 68K refers to the setting of the pot.

How do i choose what input ac coupling cap value to use
The input capacitor is selected that Xc is much smaller then resistance of a cooperating resistor at the lowest frequency of operation Xc=0.1*R.
Xc=1/(2*PI*F*C)=0.16/(F*C)
C=0.16/(F*R)

Cin = 0.16/(10Hz * 47K||100K) = 680nF
 

Kermit2

Joined Feb 5, 2010
4,158


this one actually works down to 8 ohm loads but would be considered a preamp type circuit. I'm sure Audioguru would disapprove, but I've heard this one and it seems good enough for an earphone. It's not very high voltage gain. barely over unity(emitter follower arrangement) but will boost current 2 or 3 times.
 

ifixit

Joined Nov 20, 2008
650
Hi Kermit2,

Re the circuit you posted. Q1 should be an NPN compliment of the PNP Q2.

Regards,
Ifixit
 

Audioguru

Joined Dec 20, 2007
11,251


this one actually works down to 8 ohm loads but would be considered a preamp type circuit. I'm sure Audioguru would disapprove, but I've heard this one and it seems good enough for an earphone. It's not very high voltage gain. barely over unity(emitter follower arrangement) but will boost current 2 or 3 times.
Your circuit is completely wrong because the upper transistor is connected with backwards polarity. Its voltage loss is 36 times!

I corrected it by replacing the upper transistor with an NPN type and changing the 120 ohm emitter resistor with 1.2 ohms but then it still has a signal loss of 6.7 times.

If you reduce the value of the 5k input resistor then it will have less signal loss but its input impedance will be very low.

Your capacitor values are also completely wrong.
 

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Thread Starter

curry87

Joined May 30, 2010
101
When is a preamp transistor circuit classified as a pre amp is it purely the difference in output power ?

The circuit that kermit2 posted and audioguru fixed how is the volume controlled on the earphones ?
 

Audioguru

Joined Dec 20, 2007
11,251
When is a preamp transistor circuit classified as a pre amp is it purely the difference in output power ?
A preamp can drive the fairly high input impedance (about 10k ohms) of a power amp. A power amp can drive the fairly low impedance of headphones (32 ohms to 600 ohms per ear) or a speaker (2 ohms to 8 ohms).

The circuit that kermit2 posted and audioguru fixed how is the volume controlled on the earphones ?
You must add your own volume control at the input.
 

Kermit2

Joined Feb 5, 2010
4,158
Edited for un-needed rudeness.

possibly a misprint, but it functions exactly as described in the book. try simulating it EXACTLY as it is drawn. LTspice shows it working fine. Not perfect, but fine.
 
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Audioguru

Joined Dec 20, 2007
11,251
WARNING: The two people who have posted that my circuit is wrong should reconsider their posts.
A major marketer of electronics has included this particular design in a preamp portion of their audio output section.
IMPOSSIBLE!
One very old transistor is connected upside down!
My simulation shows that it has severe attenuation (not amplification) and has severe distortion. I corrected your emitter-follower buffer.

My attachment did not upload before so it is here:
 

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Kermit2

Joined Feb 5, 2010
4,158
"a USNavy prefered circuit."

I copied it directly from the schematic into spice and when I pressed the run button it seemed to do what the book claimed it should.

Your version of the circuit in spice is different than what I made. Try it exactly as I entered it. perhaps it is a quirk in spice combined with a misprint in the book. whatever it is, I have no problem with its operation in LTspice. I selected it precisely because it is DIFFERENT from what one would expect to be a working circuit.
 
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Audioguru

Joined Dec 20, 2007
11,251
Kermit2,
Your circuit is completeley WRONG!
Post its link from the USNavy or somewhere else.
 

THE_RB

Joined Feb 11, 2008
5,430
Audioguru, I believe he is deliberately baiting you with a circuit that can never work (as you well know).

If he had any proof that a "major marketer of electronics" used that circuit he would have posted the proof here instead of his rude words.

Moderators- please keep an eye on Kermit2. :eek:
 

Thread Starter

curry87

Joined May 30, 2010
101
Can the electret mic be damaged if the impedance between the + side of the mic and the coupling cap to gnd is too low ?
 
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Audioguru

Joined Dec 20, 2007
11,251
Can the electret mic be damaged if the impedance between the + side of the mic and the coupling cap to gnd is too low?
The coupling cap from an electret mic does not pass the signal to ground. Instead it passes the signal to the fairly high input impedance of the preamp.

An electret mic has a Jfet at its output that draws about only 0.5mA. The Jfet can be damaged if the supply voltage to its external drain resistor is too high. I feed the electret mic from a 9V supply that has a series 1k resistor then a 47uF capacitor to ground as a filter. Then a 10k resistor feeds the mic from it.
 

Audioguru

Joined Dec 20, 2007
11,251
About the circuit in post 1 of this thread what does the 50nf and 10ohm in series to gnd portion of the circuit do ive seen it on the lm386 datasheet aswell but there is explanation of its purpose?
The LM380 came before the LM386 and its applications note AN-69 describes the Zoble network that suppresses a 5MHz to 10MHz oscillation. It is also a high frequency load when the inductance of a speaker causes its impedance at high frequencies to be high.
 
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