Filter for voice comms??

[Electricman2K5]

opamp gain

hi, thanks again

when you say op-amps together have a total gain of 986 from 23kHz to 53kHz. I understand that 986 is from 31.9x30.9 =986 or 29.9db but how did you work out that frequency response from 23-53Khz?

Also what does the 1M resistor do here, is it a filter acting with the 120pf cap

Thanks

 

[audioguru]

Re: opamp gain

=986 or 29.9db

No, a voltage gain or loss of 1000 is 60dB.

but how did you work out that frequency response from 23-53Khz?

A single RC lowpass filter is 3dB down at its cutoff frequency. It begins dropping at 5 times the cutoff frequency. This circuit has 3 cascaded RC filters so if you want a flat response down to 23kHz, the 3 cutoff frequencies must be around 1.5kHz.

With a gain of 31, the TL071's natural high frequency rolloff causes a cutoff frequency of about 120kHz. Since the circuit has 2 cascaded opamps, they will have a combined cutoff frequency of 60kHz which is a little above 53kHz. I cheated, the combined response at 53kHz won't quite be flat, it will be down a couple of dB.

Also what does the 1M resistor do here, is it a filter acting with the 120pf cap?

The 1M resistor establishes the opamp's input and output DC levels of 0V. Its high value doesn't load down the 100k phototransistor's collector resistor and it doesn't have any current through it (and therefore no voltage drop across it) because the TL071 has FET inputs.
The lowpass cutoff frequency of the 1M resistor and the 120pF cap is about 1.3kHz, near the required 1.5kHz frequency calculated above. :lol:

 

[Electricman2K5]

simultion of mic circuit

Hi i simulated the mic input in LT SwitcherCAD3 using the mic as an AC 0.5V source and performed an ac analysis from 1-10Mhz to see the frequency response. This is what i got, Is this what it should look like???
thanks

 

[audioguru]

Hi Electricman,
Your sim shows the open-loop gain and frequency response of the opamp without feedback and without the caps. As they usually do, your sim screwed-up! :lol: :lol: :lol: :lol:

 

[Electricman2K5]

frequency response

Do you think this frequecny response is ok for the amp cct you suggested for the receiver (using 2 opamps), the opamp parameters are probably not exactly the same as the TL071/2 but it seems ok.
thanks

 

[audioguru]

Hi Electricman,
The frequency response of -3dB from 1200Hz to 270kHz is fine for your FM modulated IR link and its 38kHz carrier.
Where's the 61dB of gain?

 

[Electricman2K5]

+9v and -9V supply from 9v battery?

Hi I want to get -9V to power the PLL currently im using 2 9v batteries with a centre tapping as ground..but i only want to use 1 device for space reasons.

Can I make -9V by passing +9v through it through an inverting opamp terminal????

 

[Nigel Goodwin]

Re: +9v and -9V supply from 9v battery?

Hi I want to get -9V to power the PLL currently im using 2 9v batteries with a centre tapping as ground..but i only want to use 1 device for space reasons.

Can I make -9V by passing +9v through it through an inverting opamp terminal????

No!, the opamp requires a -9V supply to produce a -9V output.

 

[Electricman2K5]

i see

Any suggestions?

Thanks

 

[Nigel Goodwin]

Re: i see

Any suggestions?

How about running your opamps off +/- 4.5V instead?.

 

[Electricman2K5]

-9v

I have a PLL that needs -9v I need to use 9V batteries. I can use it with split supply but that means 0nly 4.5V it needs 5V min. Its for a reciever so the gain is important. With opamps At 4.5V ill wont really have much room for battery degradation, a 9v battery will easily read only 8V or so within a week of use!!!
I ordered a (free sample!!) of a MAX1440 switchd cap voltage inverter will invert +9v to -9v supply , around 95% efficient, ive never used one but I hope it can do the job!
thanks

 

[Nigel Goodwin]

Re: -9v

I have a PLL that needs -9v I need to use 9V batteries. I can use it with split supply but that means 0nly 4.5V it needs 5V min. Its for a reciever so the gain is important. With opamps At 4.5V ill wont really have much room for battery degradation, a 9v battery will easily read only 8V or so within a week of use!!!
I ordered a (free sample!!) of a MAX1440 switchd cap voltage inverter will invert +9v to -9v supply , around 95% efficient, ive never used one but I hope it can do the job!
thanks

As long as your power requirements are low, it should be fine.