Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Convert bipolar Op-amp schematic to single source Op-amp

Status
Not open for further replies.

Increazon

New Member
I want to build Colpitts Oscillator Op-amp Circuit from here **broken link removed**
In the schematic it uses classic Op-amp 741 with 2 voltage sources. I have only Op-amp LM358 which using single voltage source.
What changes i need to do to schematic to make it work.

Imega from website:
**broken link removed**
 
ANY opamp can use a single positive supply if its (+) input is biased at about half the supply voltage and input, output and feedback coupling capacitors are used.
ANY opamp can use a positive and negative supply when its (+) Input is at about 0V (half the supply voltage) as shown in the circuit you found.

The lousy old LM358 dual opamp has crossover distortion that messes up a sinewave and it has trouble with frequencies above 2kHz. Also, you should properly disable the unused opamp so that it does not cause interference. The LM358 and any other opamp works fine with a dual polarity supply.
Why not use a modern higher frequency opamp?? Most modern audio opamps work very well up to 100kHz.

The circuit you found uses a 52 years old lousy old 741 opamp that has trouble with frequencies above 9kHz so it probably will not work.
 
To answer your question ... This is relatively simple because the only circuit components connected to GND are capacitors.

1. Add two 10K resistors in series between VCC and GND. R4 from Vcc to the midpoint, and R5 from the midpoint to GND. This establishes a reference voltage midway between your two power rails. We'll call the midpoint voltage VGND.

2. Add a 0.1 uF ceramic and a 10 uF electrolytic in parallel across R5. This decreases the AC impedance of VGND so it appears as a real ground at the oscillator frequency. BTW - WHAT is the desired oscillator frequency? As noted above, the LM358 does not perform well at high frequencies.

3. Disconnect the opamp non-inverting (+) input from GND, and connect it to VGND.

If the other half of the LM358 is available, you can use it to buffer the VGND. If you want to leave it unused ...

4. Connect the output to the inverting (-) input.

5. Connect the non-inverting input to VGND.

ak
 
A tutorial using the same parts says the frequency is 10.8kHz which the LM358 will have lots of trouble doing it.
 
I need frequency 600 kHz. My tank circuit two 2*6800pF and 10 uH
tanksm.jpg
 
I need frequency 600 kHz. My tank circuit two 2*6800pF and 10 uH
View attachment 122519

That is like asking a VW Beatle to perform like an F1 racer. You'll need to do some research if you want the op-amp solution at 600kHz (your current op amp will even have trouble with 60kHz... or even 20kHz). I suggest a transistor solution for your Colpitts oscillator.
 
Last edited:
Ok, i had achieved working Colpitts oscillator schematic.
The frequency become much lower i want - 476 kHz.
But i still can catch it on chine receiver at MW 150-285 kHz.
Next i will make AM modulation usint transformer with diodes.
ac_worksm.jpg

ac_work_sch.JPG
 
Look up gain bandwidth product and slew rate on op amps, they'll tell you how high you can go freq wise.
Limiting the amplitude to an extent increases the max freq.
I havent done any research but you might be better with an op amp like a Ne5532 or Tlc272.
C2 and C3 on your transistor schem look too big, and L1 might be a little small.
If you can settle for a triangle or square wave you could use a 7555.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top