does someone know the function of R6,R8 and R9?Is it just current limiting

[Lars1408]

http://www.circuitstoday.com/am-transmitter-circuit

 

[Chemelec]

It Doesn't look like a very good design.

R9 Supply Current for Mic.
R8 Collector resistor for Q2 or just a current limit.
R6 Bias Resistor for Q2

I Think they put the Battery on the Wrong Side of the Schematic!

 

[Chemelec]

In My opinion, This would make more sense.

 

[daisyyyyyy]

Is C6 wrong position too? In most of the circuits, it is at the side of B1, but I have no idea why

 

[Lars1408]

In My opinion, This would make more sense.

View attachment 30499

Thanks for the response....but i dont see what changes if you switch the supply to the right? isnt everyting parallel?

 

[Chemelec]

No C6 is Correct, It Filters and Stabilize the Voltage for Q1 and the Mic.
But if you have Long wires between the battery and the Circuit, you could add another cap across the battery side.

 

[Chemelec]

Thanks for the response....but i dont see what changes if you switch the supply to the right? isnt everyting parallel?

Putting the Battery on the RIGHT Side Allows for Much More Power to the Output Transistor.

With the Original Circuit, the Output transistor will get Almost NO Power because of R8.
And the Microphone Side does Not require High Power.

 

[BobK]

I agree. The R8 and C6 are to isolate the amplifier from the output stage. That only works if the battery is placed as Chemelec has done.

Bob

 

[Kiwi]

Looks like there are two C5's connected to Q1.

 

[Chemelec]

Looks like there are two C5's connected to Q1.

Yep, They Goofed on there Numbering.

But the next thing is Does this circuit even work?
Without actually Building and Testing it, No One Knows for sure.

 

[Audioguru]

They also goofed on the volume control that will do almost nothing when it is a rheostat like that.
Its value should be about 20k ohms with a logarithmic audio taper and a capacitor on both sides of it since it will be grounded.

 

[Chemelec]

Circuit is Probably designed by a 12 year old.

 

[Audioguru]

The mic preamp transistor is biased wrongly so when speaking not even very loud it produces awful distortion:

 

[Lars1408]

And how do you know what the amplification is of the circuit?

 

[Audioguru]

When you learn about how a transistor works then you learn that the voltage gain is the ratio of the collector resistor in parallel with the load resistance, divided by the unbypassed emitter resistance. Your circuit does not have any unbypassed emitter resistor so the emitter resistance of the transistor is used in the calculation for voltage gain.

If the transistor is biased correctly so that its output voltage can swing equally up and down then its collector voltage is about 5V then its collector and emitter current is (9V- 5V)/10k= 0.4mA. The emitter resistance is calculated to be 26/0.4= 65 ohms then the voltage gain will be 10k/65= 154 times.

The mic preamp transistor in my FM transmitter has a voltage gain of about 18 times because its emitter resistance is unbypassed and it is very sensitive. Yours would pickup sounds a km away full of noise if it is biased correctly but the value of R9 in your circuit is too low so it reduces the level from the electret mic.

 

[Lars1408]

When you learn about how a transistor works then you learn that the voltage gain is the ratio of the collector resistor in parallel with the load resistance, divided by the unbypassed emitter resistance. Your circuit does not have any unbypassed emitter resistor so the emitter resistance of the transistor is used in the calculation for voltage gain.

If the transistor is biased correctly so that its output voltage can swing equally up and down then its collector voltage is about 5V then its collector and emitter current is (9V- 5V)/10k= 0.4mA. The emitter resistance is calculated to be 26/0.4= 65 ohms then the voltage gain will be 10k/65= 154 times.

The mic preamp transistor in my FM transmitter has a voltage gain of about 18 times because its emitter resistance is unbypassed and it is very sensitive. Yours would pickup sounds a km away full of noise if it is biased correctly but the value of R9 in your circuit is too low so it reduces the level from the electret mic.

ok thank you