The input ESD diodes might clamp the negative swing.
An opamp like this, with a resistor to ground and output from the supply rails, makes an (almost) ideal class-B zero-crossover current splitter. Roughly like this:
-- John Larkin Highland Technology, Inc lunatic fringe electronics
On Sunday, April 9, 2017 at 12:00:53 PM UTC-7, John Larkin wrote: ...
...
That's not too goofy - the venerable NE570/571 uses a similar technique for its rectification stage.
kevin
OK, this is not so goofy.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Nice project. See comments under the AC Voltmeter heading on the 535-A tube (V301): "The voltmeter uses a unique calibrated tube (535-A) especially made for Boonton. It has a specific linearity to match the Q Voltmeter scale and uses a very high grid input resistance (100M ohms)...."
Looking at the schematic, the meter M301 is measuring the cathode current through R302, to M301, to a balancing maze of resistors and pots. The plate is at AC ground (C301) which seems rather odd. Oh, it's an "inverted triode" circuit, where the function of the grid and the plate are interchanged: Notice the high voltage (150v) on the grid through the resistor maze. This derangement is common in electrometer service and a few ancient Geiger counters, where sky high input impedance is important.
The usual scheme for transistorizing a tube circuit is to use an N-channel JFET, but I don't think it will work here. Putting 150V between the gate and the source will produce smoke, not signal.
Incidentally, have your rubber gloves and 99% alcohol bottle handy when working with electrometer tubes. If you get any sweat from your fingers on the tube, it will prefer sending current through your fingerprint instead of the going inside the tube. Handle the tube with gloves and be prepared to clean the tube with the purest alcohol you can obtain. It's much like working at high voltages.
Hmmm... now that I think of it, you might be able to fix your Boonton
260A by simply cleaning the tube.-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
No, it's just a cathode follower.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Oops. That's all wrong. I got lost trying to follow the wires around the tube. It's not an "inverted triode" amp. It now looks more like a cathode follower, with the meter reading the average cathode current.
However, I still believe that substituting an electrometer tube for the 535-A is the easiest way to fix the Q-meter. My comments on sky high imput impedance (note that R201 = 100Meg) and cleaning the tube still apply.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
Here is one you might like. I've been using fast peak detectors since
1967, and this is one of my favorites. It uses a Szlikai pair,and handles about 5mV to 2V pk. It runs up to 50MHz, but you can push it furher with faster transistors. It starts to go nonlinear below about 5mV.You can opimize the response for your specific application. For example, you can increase the dynamic range by deleting R5 and connecting R7 to a negative supply. It has thermal drift, but there are a number of ways around that. See
SW
I never owned an electronics company, and I never will, but if I did, I'd have some instructions for the gardening service about the flower beds and a certain pattern of a certain variety of flower...
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com I'm looking for work -- see my website!
Looks like the output has a big DC offset with a big tempco, and it's nonlinear below about 100 mV p-p. Output is zero below about 25 mV p-p.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
I was going to say the only circuit I know uses an opamp.. but with the diode(s) in the feed back loop. precision rectifier, I think.
George H.
Post your analysis.
My posted LTspice analysis shows the response below 5mV pk.
Nonlinearity starts below 5 mV. Change the parameter Vpk to show results up to 2V pk.
DC offset is similar to other posts.
Tempco is discussed in Wikipedia. About the same as a single diode, which is used in other proposals.
I am surprised you can be so wrong.
SW
As a true believer in LTspice, you have just demonstrated the "down the path to error" that simulators can lead novices. Sorry :-( ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| STV, Queen Creek, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
Thinking outside the box... producing elegant solutions.
I ran yours.
I did that. It sure flatlines at low AC levels. Diode detectors go square-law at low amplitudes.
Not my PNP-NPN circuit! You have about 2 volts DC out with no AC input.
Mine cancelled.
Don't be a jerk. That's way too common.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
I mostly believe the sim. The diode goes square-law below about 100 mV p-p. This ain't subtle.
That's his sim, changed only to run the input at 2 volts p-p max.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
I showed the response below 5 mV pk. You apparently cannot understand SPICE.
The offset depends on the bias voltage to the NPN, exactly the same as other circuits. You can change it as desired, and offset it with a separate bias circuit exactly as in other circuits.
I don't see any LTspice analysis. Please show your response from 5mV to zero at 50 MHz
You have not proven your observations or your claims. Please do so.
Though I'm never one to defend Larkin (I enjoy watching him trying to finagle out from under his own errors)... you'd best retreat and review your circuit, and apologize for your snarkiness. ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| STV, Queen Creek, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
Thinking outside the box... producing elegant solutions.
You can't tell anything about the linearity at 5 mV with 2V p-p input.
I showed the linearity with 5 mV pk. Post your plot with 100 mV pk input and tell us what your problem is.
Why not? Your sim sweeps through zero volts. And LT Spice zooms nicely.
The output is clearly flatlining. It has to. Set Vpk to 0.1 and you'll see the same thing, even more dramatically. The output will be pointy on top and parabolic on the bottom.
Another interesting thing about that circuit is that, at zero and low signal levels, R1 gobbles all the available Q1 collector current, so Q2 remains off. That makes the low-level linearity even worse. Q2 is useless.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.