I was searching for a high speed comparator, and soon realized that there seem to be more high performance op-amps out there than comparators.
If you don't need open-collector TTL outputs, are there any advantages to using a hi-speed comparator vs a high-speed opamp connected as a comparator?
Comps seem to have lower gain (3V/mV) than the open-loop gain of op-amps, and also have lower input resistence than opamps.
One question:
If you don't use neg feedback on an OpAmp (but maybe use a bit of pos FB for hysterisis), is the (-) input still a virtual ground? or does it become high-impedance, lke the (+) input?
Joseph
If you don't need open-collector TTL outputs, are there any advantages to using a hi-speed comparator vs a high-speed opamp connected as a comparator?
Comps seem to have lower gain (3V/mV) than the open-loop gain of op-amps, and also have lower input resistence than opamps.
One question:
If you don't use neg feedback on an OpAmp (but maybe use a bit of pos FB for hysterisis), is the (-) input still a virtual ground? or does it become high-impedance, lke the (+) input?
Joseph
Op-amps and comparators are basically the same.
Op-amps were from beginning made for being comparators
in digital applications.
They could be '0' or '1'.
Op-amps compare both their inputs, as you know.
If your intended use is not very special or extreme you can use op-amp.
Your choice of op-amp or comparator is from what your need is.
Can be from very simple low cost IC to higher precision chips.
Low cost Op-amp LM324 can for example be used as 4 comparators
with only 0.7mA total supply at +-1.5 to +-16 Volt.
http://www.national.com/pf/LM/LM324.html
Op-amps were from beginning made for being comparators
in digital applications.
They could be '0' or '1'.
Op-amps compare both their inputs, as you know.
If your intended use is not very special or extreme you can use op-amp.
Your choice of op-amp or comparator is from what your need is.
Can be from very simple low cost IC to higher precision chips.
Low cost Op-amp LM324 can for example be used as 4 comparators
with only 0.7mA total supply at +-1.5 to +-16 Volt.
http://www.national.com/pf/LM/LM324.html
Joe,
To answer your first question; op-amps and comparators are similar but the not same. You can use an op amp for a comparator but not exactly vice-versa; comparators do not have the internal integrator that is key to an op amp. If you need speed, the comparator is the way to go. This is why you don't see gain and bandwidth figures in comparator specs.
Comparators in general have not the development in recent years as much as op amps have. ADC convertors and microprocessors with ADC's take over their former applications.
If using an op amp for a comparator, greater off the shelf accuracies are attainable but usually with some loss in speed. An op amps output will "slew" AFTER the internal integrator has charges or discharges... to maximize speed you must use zeners (or other methods) around the op amp to ensure that the outputs don't saturate.
For your second question... both inputs of an op amp have similar impedances... virtual ground is created by the circuit it sits in... and here again if the output is saturated... the virtual ground is not there.
Post a scematic of what you would like to do...
Cheers,
To answer your first question; op-amps and comparators are similar but the not same. You can use an op amp for a comparator but not exactly vice-versa; comparators do not have the internal integrator that is key to an op amp. If you need speed, the comparator is the way to go. This is why you don't see gain and bandwidth figures in comparator specs.
Comparators in general have not the development in recent years as much as op amps have. ADC convertors and microprocessors with ADC's take over their former applications.
If using an op amp for a comparator, greater off the shelf accuracies are attainable but usually with some loss in speed. An op amps output will "slew" AFTER the internal integrator has charges or discharges... to maximize speed you must use zeners (or other methods) around the op amp to ensure that the outputs don't saturate.
For your second question... both inputs of an op amp have similar impedances... virtual ground is created by the circuit it sits in... and here again if the output is saturated... the virtual ground is not there.
Post a scematic of what you would like to do...
Cheers,
Hi lineup,
Comparators have a high impedance between the + and - inputs. You can stick one input at +8V and the other at ground, as long as you don't exceed supply voltages or input range. An op amp will not be happy with you if you do this. A bipolar input op amp may break down reverse b-e with these voltages, a comparator will not. Comparators do not have differential inputs as a rule.
If you want to use an op amp as a comparator, you can as long as you watch input voltages and instantaneous voltages at the input pins. Some sort of current limiting may be in order.
-Chris
Hi poobah: X post with you, but I'm right
Comparators have a high impedance between the + and - inputs. You can stick one input at +8V and the other at ground, as long as you don't exceed supply voltages or input range. An op amp will not be happy with you if you do this. A bipolar input op amp may break down reverse b-e with these voltages, a comparator will not. Comparators do not have differential inputs as a rule.
If you want to use an op amp as a comparator, you can as long as you watch input voltages and instantaneous voltages at the input pins. Some sort of current limiting may be in order.
-Chris
Hi poobah: X post with you, but I'm right
thanks for info, anatech and poobah
this will give the topic poster info he wants
I imagine dedicated comparators use 'switch transistors'
with very short rise and fall time
Another thing may be full rail voltage at output
( even if there are some op-amps with rail-to-rail output, too )
A parameter often seen in comparators is 'settling time'
I have only a slight thinking of what this may be,
but do not know exactly
this will give the topic poster info he wants
I imagine dedicated comparators use 'switch transistors'
with very short rise and fall time
Another thing may be full rail voltage at output
( even if there are some op-amps with rail-to-rail output, too )
A parameter often seen in comparators is 'settling time'
I have only a slight thinking of what this may be,
but do not know exactly
Comparators are not required to operate in closed loop, thus they don't require any internal frequency compensation (at the expense of reduced slew rate). Also, op-amps may have higher open-loop gain at low frequencies, but comparators have higher gain at high frequencies, due to the absence of compensation.
As it has been mentioned, op-amps have evolved a lot in the past decades and hundreds of new models have appeared as opposed to what happened with comparators, but these facts are quite easy to understand. Lets take a LM393 dual comparator and a uA741 dual op-amp as a reference, both are very old devices from 1970s, but while the op-amp is very slow and too noisy for most today's applications, the comparator is fast and is still fine for new designs (my LM393 are swinging from rail to rail in 200ns or so provided adequate input overdrive and output bias).
Don't underestimate devices just for being old. There is still a lot of old stuff regarded as industry standard because it still fits today's needs quite well. Personally, I use a lot of LM393, CD4000 CMOS gates, TL431, BC546/556, BD139/140, LM358, 1N4007, 1N4148 etc... and I like these devices.
PD. Using op-amps as comparators works but produces poor performance.
As it has been mentioned, op-amps have evolved a lot in the past decades and hundreds of new models have appeared as opposed to what happened with comparators, but these facts are quite easy to understand. Lets take a LM393 dual comparator and a uA741 dual op-amp as a reference, both are very old devices from 1970s, but while the op-amp is very slow and too noisy for most today's applications, the comparator is fast and is still fine for new designs (my LM393 are swinging from rail to rail in 200ns or so provided adequate input overdrive and output bias).
Don't underestimate devices just for being old. There is still a lot of old stuff regarded as industry standard because it still fits today's needs quite well. Personally, I use a lot of LM393, CD4000 CMOS gates, TL431, BC546/556, BD139/140, LM358, 1N4007, 1N4148 etc... and I like these devices.
PD. Using op-amps as comparators works but produces poor performance.
Good point Eva,
I just finished a design for long production... I had the choice of high voltage CMOS or a small uP with bipolar buffers. I had to choose for the new uP because the manufacturers won't promise the future of HV CMOS. Time will tell if I made the right choice.
But you are very right... some old devices were made/designed so well in the first place... there is not much much to improve.
I just finished a design for long production... I had the choice of high voltage CMOS or a small uP with bipolar buffers. I had to choose for the new uP because the manufacturers won't promise the future of HV CMOS. Time will tell if I made the right choice.
But you are very right... some old devices were made/designed so well in the first place... there is not much much to improve.
Thanks, dear members.
Although i don't have a schematic yet, the application is a switch to connect the power output transistors of a power amplifier to a higher voltage supply during audio peaks (you call this class G or class H I think).
So one input of the comparator has a reference voltage to it (DC). The other, has an audio signal coming to it.
The output would feed a MOSFET driver, driving a couple of large MOSFETS in parallel.
Because there are already some propagation delays imposed in the circuit by the mosfets and driver, I really can't afford to have too much delay in the comparator. 100ns is too much.
I was looking at the LM360, and the specs look right. But I don't have an LM360, and I do have an LMH6655. Everything about the opamp seems to be better than the comparator, in my naive evaluation.
(200V/uS slew rate, 180mA output current, 250MHz bandwidth...)
Joseph
Although i don't have a schematic yet, the application is a switch to connect the power output transistors of a power amplifier to a higher voltage supply during audio peaks (you call this class G or class H I think).
So one input of the comparator has a reference voltage to it (DC). The other, has an audio signal coming to it.
The output would feed a MOSFET driver, driving a couple of large MOSFETS in parallel.
Because there are already some propagation delays imposed in the circuit by the mosfets and driver, I really can't afford to have too much delay in the comparator. 100ns is too much.
I was looking at the LM360, and the specs look right. But I don't have an LM360, and I do have an LMH6655. Everything about the opamp seems to be better than the comparator, in my naive evaluation.
(200V/uS slew rate, 180mA output current, 250MHz bandwidth...)
Joseph
It can (might) be done... you will have to take some measures to ensure that the output is not allowed to saturate... otherwise there will be significant delay between "a comparison" and a cooresponding output change. This will become easier to understand once you have a schematic.
And as :King:Anatech points out; check the input specifications carefully... look for "differential input voltage" and be sure not to exceed it. Doing so may not burn the amp, but the output could reverse or other crazy things could happen.
A comparator might still be your best choice though... there are ways to "anticipate" the signal and allow for some delay.
Is this all about improving dynamic headroom without the additional heat?
And as :King:Anatech points out; check the input specifications carefully... look for "differential input voltage" and be sure not to exceed it. Doing so may not burn the amp, but the output could reverse or other crazy things could happen.
A comparator might still be your best choice though... there are ways to "anticipate" the signal and allow for some delay.
Is this all about improving dynamic headroom without the additional heat?
Hi Joseph,
Can you do the switching in the analog domain? Most other designs use transistors. There isn't much need to be faster than the audio frequency. Unless you will go mids or highs only the large swings will be lower frequency notes. Similar to Carver, but he held his rail up for a minimum time.
Another consideration is that abrupt switching will induce spikes in the output. Avoid.
-Chris
Can you do the switching in the analog domain? Most other designs use transistors. There isn't much need to be faster than the audio frequency. Unless you will go mids or highs only the large swings will be lower frequency notes. Similar to Carver, but he held his rail up for a minimum time.
Another consideration is that abrupt switching will induce spikes in the output. Avoid.
-Chris
Hi poobah,
Carver did it right. NAD, well okay, they got the effect. I was never impressed with the quality of NAD. The same can be said for others.
Properly switching the supplies requires some engineering to get right. More trouble for DIY, okay for a big lab such as Carver corp. I'd say there were other factors that determined the Carver sound that had little to do with supply switching. On earlier products you could certianly hear it.
-Chris
Carver did it right. NAD, well okay, they got the effect. I was never impressed with the quality of NAD. The same can be said for others.
Properly switching the supplies requires some engineering to get right. More trouble for DIY, okay for a big lab such as Carver corp. I'd say there were other factors that determined the Carver sound that had little to do with supply switching. On earlier products you could certianly hear it.
-Chris
Oh... I was never of a fan of NAD. As far as that goes, I wouldn't like God's best amp if it were named GROIN...
I was just thinking there might be ideas for Joe. I've been pondering this... just how do you know a big power demand is coming the moment before it does?
Maybe roll off the highs (only in the trigger circuit) and set the trigger at 50-70% percent of the "regular" rail voltage?
And
Chris is absolutety right; you'll have to kick in the boost supply REAL smooth to avoid audible trash going into the drains, collectors, plates etc...
hmmm... I think part of your answer lies in there Joe... this is interesting, lemme do some calcs...
I was just thinking there might be ideas for Joe. I've been pondering this... just how do you know a big power demand is coming the moment before it does?
Maybe roll off the highs (only in the trigger circuit) and set the trigger at 50-70% percent of the "regular" rail voltage?
And
Chris is absolutety right; you'll have to kick in the boost supply REAL smooth to avoid audible trash going into the drains, collectors, plates etc...hmmm... I think part of your answer lies in there Joe... this is interesting, lemme do some calcs...
Hi poobah,
Just drive the "commutator" with the input signal raised 5 ~ 10 V above the waveform. As the signal amplitude increases, the supply voltage starts rising above a certain level. The higher level will predict the audio signal in a way.
This is really cool to watch in operation using a couple 'scopes.
-Chris
Just drive the "commutator" with the input signal raised 5 ~ 10 V above the waveform. As the signal amplitude increases, the supply voltage starts rising above a certain level. The higher level will predict the audio signal in a way.
This is really cool to watch in operation using a couple 'scopes.
-Chris
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