amplify the power (infrared)

[audioguru]

Provide a high resistor value for the phototransistor's collector resistor, a high impedance load for it from the high frequency opamp's input impedance and more gain from the opamp.
Then the range will be plenty with your two IR LEDs.

[ryanleung]

how to check the opamp which can satisfy the 38kHz operation?

[windoze killa]

I used two IR LEDs already.

Try 4.

[audioguru]

how to check the opamp which can satisfy the 38kHz operation?

I already told you that the LM324 is lousy and showed a graph from its datasheet.
I also said that the TL07x family of opamps performs well up to 100kHz.
The TL07x family of opamps is low noise, low distortion, wideband and inexpensive.

Here is how the opamps compare:

[audioguru]

Doubling the number of IR LEDs won't make much difference when the receiver circuit results in such a high loss instead of a high gain.

[ryanleung]

ok
thank you~~~

[ryanleung]

oh....sorry.....
I have some question about my transmitter.
I constructed the receiver already and I used the remote control to test the receiver.  The receiver can receive the signal.  If I used my transmitter to transmit the AM signal, the receiver can receive the signal but the signal is small.  how can i modify the transmitter?  the input signal only have about 1Vp-p, should i amplify the signal first?

[audioguru]

I inverted the black background of your schematic.
The value of R7 in your transmitter is so high that the modulator's DC input voltage needs to be -144.5V to bias Q2 into a linear operating region. Also, a very high modulating voltage swing would be needed.

If R7 is 39 ohms and the modulating source is capable of driving such a low impedance, when the modulating input DC voltage is 0V then Q2 would be biased half-way. An input signal swing of 2.4Vp-p would fully turn on and off the IR LEDs. The carrier signal from Q1 to Q2 would need attenuation because then Q2 would have some gain.

I don't see how modulating the emitter of the transistor creates amplitude modulation. It seems that the carrier would just have its top or bottom clipped by the emitter's modulation.

[ryanleung]

umum......
How can I modify it now?
Thanks

[audioguru]

I think the base of the oscillator should be modulated then it can connect to a common-emitter LED driver transistor.
An oscilloscope will show you what the transmitter is doing.

[SM2GXN]

Hi audioguru!

I think it will create AM and two sidebands if you inject a sine wave into the emitter,
if I don't remember wrong just about same configuration is used in RF but as collector load a tuned L/C circuit.
What do you think audioguru? 

[SM2GXN]

By the way audioguru, have ever tried to simulate an AM circuit in switchercad?

[ryanleung]

I think the base of the oscillator should be modulated then it can connect to a common-emitter LED driver transistor.
An oscilloscope will show you what the transmitter is doing.

sorry, I don't know your mean. can you draw a circuit to me, thanks!!!

[audioguru]

This AM modulated IR transmitter circuit should work: