Voltage follower with a step function (need a design for one)

What you have drawn will IMHO do need a high bandwidth, actually an infinite BW. And with 20kHz this approaches pretty much the limit of a normal opamp, especially when you want no hysteresis. To switch 20kHz in say 0.1% of its period means 50ns and if a comparator will need already say 40ns to detect the threshold x-ing, you have to switch 2V to gnd in 10ns, which is possible. But then the O/P signal will need a really fast opamp buffer with this 200V/us slew.

Another thing is you mention AC signals. Do you want a symmetrical threshold, you have not drawn anything to the left of your y-axis. What should happen with negative values?

You could use a comparator with an adjustable voltage on one input, the circuit input voltage on the other, and the comparator output going to the control input of an analog switch, like a 4066. The analog input to the switch is also connected to the circuit input, and the analog switch output is connected to a resistor, say 10kohm, to ground, and to the circuit output. You might be able to use just an FET as the analog switch, and an opamp section as the comparator; then with a quad opamp you could have voltage followers on input and output, and one section left over.

-- john

-- john

You could "effect" it like so- but there are details in stability to attend to, and the output amp clamps low at 0V: View in a fixed-width font such as Courier.

. . . . .-------------. . | | . | | . | /| | . | /-|-- . +---| -+--------> Vout . +-------|-/ . | |/ . [R] . | . --- . .

Rocky, how about this:

just clamp the input to the op amp to zero until the input is above 2 volts, then let it track the input. For example, run the input through a couple of series resistors, and put a clamping transistor driven by a voltage comparator at the node between the two resistors. The clamping transistor is driven to saturation by a second op amp when the input is less than two volts, and driven off when 2 Volts is exeeded.

See "DeadZone.pdf", a two-sided equivalent to your problem, on the S.E.D/Schematics page of my website.

But there are many ways to implement such circuit functions... surf on "piece wise linear" or "breakpoint analysis"... commonly taught in undergraduate EE classes.

...Jim Thompson

This should be trouble free using a single-supply type OA: View in a fixed-width font such as Courier.

. . . . .---------------. . | | . | | . | /| | . | /-|--' . +---|

Thank you everyone for you contributions to my question. In summary I'll probably use either John Fields' or Tony's circuit because they are single chip solutions. They are both similar in their approach, just that John used a comparator in place of Tony's transistor to pull down the input when 0

Hi John, I was just about to implement your suggested circuit when I came across something I don't understand. When Vin >2V, does the output from the LM392 comparator swing to the comparator's positive supply rail? I thought that's how a comparator worked - basically an opamp that allows rail to rail output swing. If however the comparator's output impedance went high (like a transistor when switched off) when its positive input voltage >neg. input voltage then that would make sense, but because I can't see how it would, then surely we need the opamp to drive a transistor as Tony suggested above.

While I think about it, is there any reason why I need to use comparator IC instead of say an LM158/LM358 opamp ? I justify my reasoning later on but I ask this question because I firstly don't have any comparators lying around in my spare parts box to prototype the circuit, and secondly I'd like to keep the I.C. count low if possible (I know the LM392 is a good choice with comparator+ompamp in the one pkg, but I'd need to mail order the bit through Farnell or Radio Spares which would unnecessarily bump up the cost for me.) Anyway, the datasheet for an LM358 opamp says: "In the linear mode the input common-mode voltage range includes ground and the output voltage can also swing to ground, even though operated from only a single power supply voltage.". Additionally, because I only need to run at about 20Hz, even if I do drive the output into the

regards Rocky

Hi John, thanks again for your timely and valuable information, I am very appreciative. regards Rocky.

In article , John Fields wrote: [....]

If the max signal voltage is under about 7V, swap Q1's E and C legs.

The circuit John drew shows a transistor in common emitter configuration, (which must be an NPN tranny assuming we want the conventional current to flow from C to E). So you are suggesting a common collector config. for Vin

I see the reduction.

Reversed: Vce 2.47mV Ie 0.395mA Ib 1.280mA Ic 1.675mA

Forward: Vce 7.73mV Ic 0.400mA Ib 1.270mA Ie 1.670mA

However, that is VERY DEEP saturation case with a beta of about 0.3. If I keep the transistor saturated, but reduce the 10k resistor to 1k thus bringing the collector current up, but still seriously saturated with only a beta of 2 or 3:

Reversed: Vce 40.77mV Ie 3.960mA Ib 1.275mA Ic 5.235mA

Forward: Vce 20.07mV Ic 3.980mA Ib 1.260mA Ie 5.240mA

So when your base drive significantly dominates everything else, you can squeeze a few more millivolts out of the Vce, reversed.

Is that about it?

Jon

I downloaded LTSpice and opened up the above text as a circuit file (*.cir), when I ran it, I got the error message "Circuit: Version 4

Fatal Error: Multiple instances of "Symattr"

Do you know why? regards Rocky