invert the output a wave rectifier.

[Analog_Fan]

Hi there,



I'm looking at a wave rectifier circuit and i want the output inverted but not centered around GND, but to the power line.
that is when the outputted wave sits on the GND, the output should be the voltage of the power rail and follow the output of the opamp producing an inverted image.

I tried sticking on P/N-channel mosfets, all sorts of pnp/npn transistors configurations on the output of the opamp coupled to the power rail, using P type of devices distorted the output of the opamp, so i stuck on a voltage follower.
But somehow it doesn't or doesn't work to work.



How it's done?

 

[abbey road d enfer]

Hi there,

View attachment 107186

I'm looking at a wave rectifier circuit and i want the output inverted but not centered around GND, but to the power line.

I'm not sure I understand what you want. Inverting is easy, you just need to reverse the diode, but "centered to the power line", huh. Do you mean added to the power line voltage? You would need an adder.
Could you draw the waveform you want to achieve?

 

[Analog_Fan]

Could you draw the waveform you want to achieve?

Thank you for replying.

Well, that what wave rectifier outputs but tied to the power rail in instead of GND.
Upside down, so to speak.

I thought to believe i had a circuit made in December that could do it, but reviewing it again, it does not.

 

[abbey road d enfer]

View attachment 107202

Thank you for replying.

Well, this doesn't tell much, since we don't know where is zero volt.

Well, that what wave rectifier outputs but tied to the power rail in instead of GND.

Ok, so the reference is some DC voltage. I believe it can be done, but it takes some thinking.
You need to scale according to your needs.

 

[JohnRoberts]

View attachment 107202

Thank you for replying.

Well, that what wave rectifier outputs but tied to the power rail in instead of GND.
Upside down, so to speak.

I thought to believe i had a circuit made in December that could do it, but reviewing it again, it does not.

Image looks like half wave rectifier. Are we to presume the + power rail is the flat top of that wave form?

Does the output need to drive a low impedance?

JR

 

[Analog_Fan]

Image looks like half wave rectifier.

That's one i toke of the internet, saved me some work of cropping a browser the screenshot, saving and stuff.
But it the pretty much the same circuit I'm using (in the same simulator)
: )

I wanna drive a old skool vca, but in reverse, not sure if the PNP circuit in the drawing is hi or low impedance, i would say high impedance since it has to traverse some 15/20k trimmer + 1K resistor.



The VCA has to stay open at all times accept for incoming waves where it has to reduce to the "volume".
The VCA will not be part of the circuit i intent to build, it's a separate module.

I tried again with a NPN emitter follower, but a high value resistor on the base.
apparently it doesn't take much to saturate the the transistor.

(the actual circuit).

I though the ideal candidate would be a MOSFET, being "sensitive" to voltage, but even with a 10M resistor on the Gate it just slams up and down, not following the curvature.

 

[abbey road d enfer]

I wanna drive a old skool vca,

What is an old school VCA? What is the control law. I see you have an OTA, which requires current control, not voltage. Is it what you want? Other VCA's have different control schemes.

The VCA has to stay open at all times accept for incoming waves where it has to reduce to the "volume".

What are the "incoming waves"? DC control voltage or audio-ish signals?

The VCA will not be part of the circuit i intent to build, it's a separate module.

Is it a typical 0-5V control?

I though the ideal candidate would be a MOSFET, being "sensitive" to voltage, but even with a 10M resistor on the Gate it just slams up and down, not following the curvature.

What means "following the curvature"?

 

[Analog_Fan]

What is an old school VCA? What is the control law. I see you have an OTA, which requires current control, not voltage. Is it what you want? Other VCA's have different control schemes.

Old Skool VCA's use a PNP transistor with the base tied to GND, like Roland Jupiter 8.
Apparently if you raise the voltage on the emitter above GND, the collector syncs current into GND.

What are the "incoming waves"? DC control voltage or audio-ish signals?

LFO's

Is it a typical 0-5V control?

my ADSR outputs 10V, so my VCA's are dialed in for 10V.

What means "following the curvature"?

Well, i tried to tell that it should follow the input profile, like Sine wave.
excuse moi if it wasn't clear.

 

[abbey road d enfer]

Old Skool VCA's use a PNP transistor with the base tied to GND, like Roland Jupiter 8.
Apparently if you raise the voltage on the emitter above GND, the collector syncs current into GND.

Seems it's just one part of the VCA, actually the exponential converter that gives constant dB per volt.

LFO's.

You want the gain of the VCA to follow the amplitude of the LFO signal? But why the inversion?

my ADSR outputs 10V, so my VCA's are dialed in for 10V.

OK. So 10dB/V, correct?

Well, i tried to tell that it should follow the input profile, like Sine wave.

From you original description, you want the amplitude of signal to decrease when the LFO signal is positive, but no change when it's negative, is it correct?

 

[Analog_Fan]

I got what i want, in theories.
Turns out you need to use very high value of resistors on transistors to make the transistor follow the input of the base and producing an inverted copy.

OK. So 10dB/V, correct?

I'm not really used to see things or deal by dB.
I only know it's a exponential scale or so.
Something for later when i wanna make a VU meter.