# Need Help in modifying this circuit

#### ojou-san

Joined Jan 7, 2013
20
Hello. I am currently taking BS ECE and we are given a task to make a wireless microphone. I found this schematic while surfing on the internet: http://www.reconnsworld.com/transmit/fm_trasm.gif

#### #12

Joined Nov 30, 2010
18,224
I think you should ask your instructor to point out the sections of your text books where he taught you how to do this. Then ask him how he wants it modified in terms of frequency, range, distortion, or whatever he really wants.

#### tshuck

Joined Oct 18, 2012
3,534
The first thing to do is understand what the circuit is doing, then you can understand how to modify it.

You can see that the first transistor is an AC coupled amplifier stage - so you can modify the gain of this stage.

The second transistor is doing the modulating of the amplified AC signal from the microphone.

Have you tried stimulating this circuit? That way, you can make adjustments without risk of damaging things. Just make sure to account for losses when designing the actual circuit.

#### aws505

Joined Mar 11, 2013
59
Hey,

It looks like the amplifier is a simple two-stage audio amplifier and mixer. The microphone picks up sound and then A/C couples the sound signal through C1 to the first stage.The first stage, Q1, seems to be implemented as a Class-A (or maybe just on the verge of Class-AB -- it's DC bias is at 0.8V) amplifier for the audio signal. The audio signal is then AC coupled thru C2 to the second stage, Q2. It looks like Q2 is an oscillator whose oscillation frequency is determined by L1 and C4.

Unfortunately, I don't exactly understand the specifics of the calculations necessary to make intelligent changes. A little googling suggests that the second stage my be a Colpitts oscillator. What exactly does your instructor wish you to alter? Are you supposed to get the transmitter to work at a specific frequency? With a specific antenna? At a certain gain? The more information you provide, the more we can help.

#### Sue_AF6LJ

Joined Mar 16, 2013
45
The circuit in question isn't really a straight forward example of an FM transmitter.
Read this, when you get done you will see how that circuit works.

By the way there are more straight forward ways of generating an FM signal.
This method is less than desirable since there is also a lot of unwanted amplitude modulation created by the modulation process.
Here is a useful search page with useful links.

There are about a half dozen ways to generate an FM modulated signal.
Some are really interesting....

#### ojou-san

Joined Jan 7, 2013
20
I think you should ask your instructor to point out the sections of your text books where he taught you how to do this. Then ask him how he wants it modified in terms of frequency, range, distortion, or whatever he really wants.
Well we can modified in anything we want as long as it is not the same as that reference. btw thanks for replying!

#### ojou-san

Joined Jan 7, 2013
20
The first thing to do is understand what the circuit is doing, then you can understand how to modify it.

You can see that the first transistor is an AC coupled amplifier stage - so you can modify the gain of this stage.

The second transistor is doing the modulating of the amplified AC signal from the microphone.

Have you tried stimulating this circuit? That way, you can make adjustments without risk of damaging things. Just make sure to account for losses when designing the actual circuit.
Thanks~! Well I am trying to simulate it in multisim but I cannot find all the parts. -.-

#### ojou-san

Joined Jan 7, 2013
20
Hey,

It looks like the amplifier is a simple two-stage audio amplifier and mixer. The microphone picks up sound and then A/C couples the sound signal through C1 to the first stage.The first stage, Q1, seems to be implemented as a Class-A (or maybe just on the verge of Class-AB -- it's DC bias is at 0.8V) amplifier for the audio signal. The audio signal is then AC coupled thru C2 to the second stage, Q2. It looks like Q2 is an oscillator whose oscillation frequency is determined by L1 and C4.

Unfortunately, I don't exactly understand the specifics of the calculations necessary to make intelligent changes. A little googling suggests that the second stage my be a Colpitts oscillator. What exactly does your instructor wish you to alter? Are you supposed to get the transmitter to work at a specific frequency? With a specific antenna? At a certain gain? The more information you provide, the more we can help.
Thanks for helping Hmm Well our professor wanted us to modify the circuit but it is up to us on how to modify it. well base on what you said I think I wanted it to work in a frequency lower that 88MHz because that circuit works at 88MHz. Well if you want me to state a specific range well maybe in I wanted it to work in a frequency in the range of AM radio.

#### Sue_AF6LJ

Joined Mar 16, 2013
45
Thanks for helping Hmm Well our professor wanted us to modify the circuit but it is up to us on how to modify it. well base on what you said I think I wanted it to work in a frequency lower that 88MHz because that circuit works at 88MHz. Well if you want me to state a specific range well maybe in I wanted it to work in a frequency in the range of AM radio.
That circuit might not work that low and produce FM modulation.