hi hoy,
Improve the transistor switch off speed, better trailing edges.
E

 

If Q1 is a fast MOSFET, its Rdson will contribute much less error to the output voltage than a 3904's Vcesat.

ak

 

If Q1 is a fast MOSFET, its Rdson will contribute much less error to the output voltage than a 3904's Vcesat.

ak

Hi Analog,

Thank you
Could you please suggest one fast MOSFET.

Regards
HAri

 

Hi Ai,

Thank you very much. The output will be connecting to the Gate of a MOS.

May I know how to implement this circuit using OPAMP. I prefer OPAMP one.

I tried some opamp circuits but when the I/P frequency becomes 1Mhz the output is not proper.

Regards
HARI

Here is a fairly simple op amp circuit that will do what you need, but using an op amp at 1MHz means you have to use an op amp that can handle that frequency. That means that you will probably have to go with maybe a 4MHz unit maybe higher.

 

HI Ai,
Thank you .
Please see my circuit below .
The Opamp has

• 400MHz Gain Bandwidth Product
• 2500V/μs Slew Rate

But still, my output is slewing.You have ant thoughts why is it so



Regards
HAri

 

HI Ai,
Thank you .
Please see my circuit below .
The Opamp has

• 400MHz Gain Bandwidth Product
• 2500V/μs Slew Rate

But still, my output is slewing.You have ant thoughts why is it so

View attachment 239786
View attachment 239787
Regards
HAri

Hi,

You could try changing both 10k resistors to 1k. Many high speed amplifiers use very low impedance external parts.

Check to see that the analog switch 'on' resistance is low enough too, should be 100 Ohms or less but hopefully much less.
Also check that the op amp speed and slew rate is spec'd with a 5v only power supply too.

 

Hi Ai,
It is working after changing the resistor values. There is a delay between input and output. Please see the below pic.
May I know why high-speed amplifiers use very low impedance external parts


Regards
HAri

 

Hi Ai,
It is working after changing the resistor values. There is a delay between input and output. Please see the below pic.
May I know why high-speed amplifiers use very low impedance external parts
View attachment 239788

Regards
HAri

Hello,

The delay looks to be on the order of around 100ns. Check the spec on the analog switch you are using to see what the delay is from the control line to the switch state, and be sure to check those specs at the actual operating voltage (power supply, which may be only 5v). Sometimes the lower the voltage the slower the analog switch response. If any analog switch causes too much of a problem we may have to change that to a transistor of some type.
But is that delay a problem for the end application?

One reason high speed amplifiers use low impedance external parts (like resistors) is because of various parasitic capacitances which includes input capacitance. Since these capacitances take time to charge and discharge and some of them get their current from that passing through those external resistances, the lower the resistances the faster the capacitances charge and discharge and thus the response becomes faster. In the case of this op amp it could be a little input capacitance causing a slower than normal slew rate as the external 10k resistors were working with one of the inputs and so lowering them would cause any input and parasitic capacitance to charge and discharge faster. When the frequency goes up this becomes more important because then the charge and discharge times become comparable to the signal period. Speeding up the charge and discharge means those times become much shorter than the signal period and so the signal appears to be cleaner.
Another factor is lead length and PC board trace length. These signal paths should be kept as short as possible in order to avoid adding more inductance which also could cause problems.

 

Hi Ai,

Thank you very much for your explanation. I have a question,

Parasitic capacitance is associated with all kinds of Op_amps (Common mode and differential mode capacitance).
May I know you mean to say these capacitance are more for high-speed opamps.

Regards
HARI

 

The capacitance is not higher, but the capacitive impedance becomes lower as the frequency increases.

Bob

 

The capacitance is not higher, but the capacitive impedance becomes lower as the frequency increases.

Bob

Hi Bob,

The capacitance decreases with an increase in frequency. That is applicable to all opamps input capacitance, correct me if I am wrong.
May I know the speciality of the input capacitance of high-speed opamps.

Regards
HARI

 

The capacitance decreases with an increase in frequency.

No, it doesn't.. The *impedance* of a capacitor, any capacitor, decreases as the frequency of the signal going through it increases.

For example, a 10 K resistor to GND with a 1 pF stray capacitance across it looks like a pure resistance at low frequencies. But at video frequencies, it looks like an R-C network with a 15 MHz corner frequency. An audio signal will be completely unaffected, but a 30 MHz video signal could be attenuated by 50% (depending on the actual circuit).

There is a special type of opamp called current feedback. These components are designed for very high speed signals and use relatively low circuit impedances to nullify the effects of stray capacitance. In the example above, if the resistor is decreased to 100 ohms, the corner frequency of the R-C network moves out to 1.5 GHz, high enough to not affect the same 30 MHz signal.

Note: those are very simplistic examples to illustrate a point. High speed circuit design has many more interacting factors.

ak

 

Hi Analog,

Thank you very much.

Let me ask my question in another way.
Op-amps have two capacitance at the input.(Common mode and differential) compared to normal op-amps will these
capacitance's are higher for high speed op-amps.

Regards
HARI

 

Hi Bob,

The capacitance decreases with an increase in frequency. That is applicable to all opamps input capacitance, correct me if I am wrong.
May I know the speciality of the input capacitance of high-speed opamps.

Regards
HARI

Hi,

The design goal of a high speed op amp is to decrease the internal capacitances and of course external too when that is under manufacturing design control, but with ANY capacitance we get a delay in the charge and discharge times due to any series resistances, so in that sense the capacitance of high speed op amps will be less for those capacitances under control by the manufacturing process. The associated recommended application notes will specify typical external parts for given target design goals and that will mean low resistances for maximum performance.

So although we may see lower cap values for higher speed op amps, we also have to keep the external resistances as low as possible. There is a tradeoff though because we dont want to use up all of the available output current we need some left for the application load itself.

This is one of the nasty constraints for CPU's that have to run at very high speed (GHz ranges). As the speed increases we like to see lower capacitance values so we like to keep on die component dimensions small, but there are still resistances we have to deal with and they dissipate power which causes heat and getting rid of heat in a small area is not that easy so we reach a physical limit. We have to reduce die transistor size to keep losses down, but that does not eliminate the heat completely when we go down in size we just meet another road block, Although it may be farther down the road now it still shows up so we like to keep the resistances as low as possible.

With the op amp circuit design we dont have control over the op amp itself anymore but we do have some control over what external resistance values we can use and we keep them in line with the app notes and that almost always means lower values.

 

Hi,

The design goal of a high speed op amp is to decrease the internal capacitances and of course external too when that is under manufacturing design control, but with ANY capacitance we get a delay in the charge and discharge times due to any series resistances, so in that sense the capacitance of high speed op amps will be less for those capacitances under control by the manufacturing process. The associated recommended application notes will specify typical external parts for given target design goals and that will mean low resistances for maximum performance.

So although we may see lower cap values for higher speed op amps, we also have to keep the external resistances as low as possible. There is a tradeoff though because we dont want to use up all of the available output current we need some left for the application load itself.

This is one of the nasty constraints for CPU's that have to run at very high speed (GHz ranges). As the speed increases we like to see lower capacitance values so we like to keep on die component dimensions small, but there are still resistances we have to deal with and they dissipate power which causes heat and getting rid of heat in a small area is not that easy so we reach a physical limit. We have to reduce die transistor size to keep losses down, but that does not eliminate the heat completely when we go down in size we just meet another road block, Although it may be farther down the road now it still shows up so we like to keep the resistances as low as possible.

With the op amp circuit design we dont have control over the op amp itself anymore but we do have some control over what external resistance values we can use and we keep them in line with the app notes and that almost always means lower values.

Hi Ai,

Thank you.

Regards
HARi