Variable Attenuation for Lab Amplifier - Page 1
Forum section: electronics / projects

author: Shay
author_rank: Regular Contributor
timestamp: June 08, 2024, 10:22:32 pm
content: Hello,I am currently designing a lab amplifier for my signal generator and other general signals that I would like to amplify or buffer.Given that I am using ADA4870 amplifiers in parallel, all with fixed gain, I believe it would be beneficial to incorporate a variable attenuation option at the input. This would allow for a wider range of input voltages.I have experimented with MOSFETs and BJTs as attenuators, but they tend to exhibit high capacitance. I also tried using an optoresistor, which I constructed with a photoresistor and a super bright LED. While this solution has low capacitance and avoids the Miller effect, it lacks linearity.I am seeking advice on the best approach to achieve variable attenuation for input frequencies up to 15 MHz. Minimizing distortion is crucial, as I need to maintain the integrity of fast-rising square waves without excessive rounding of the corners.Any suggestions or tips would be greatly appreciated.Thank you.

author: Benta
author_rank: Super Contributor
timestamp: June 08, 2024, 10:39:52 pm
content: What's wrong with a potentiometer?

author: Shay
author_rank: Regular Contributor
timestamp: June 08, 2024, 11:04:55 pm
content: Quote from: Benta on June 08, 2024, 10:39:52 pmWhen I tested 10 turn potentiometers they had too much parasitics which distorted the signal

author: moffy
author_rank: Super Contributor
timestamp: June 09, 2024, 12:02:10 am
content: Quote from: Shay on June 08, 2024, 11:04:55 pmWhen I tested a smallish single turn panel mount potentiometer of 1k value I got 10MHz, with no noticeable distortion, a lower value pot will give greater bandwidth, say 200R.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: June 09, 2024, 01:47:08 am
content: Look around on analog.com for a VGA (variable gain amplifier) you’ll find reference designs. You can have linear V/V or logarithmic scale. Added benefit you can control the attenuation/gain programmatically.

author: David Hess
author_rank: Super Contributor
timestamp: June 09, 2024, 09:13:57 am
content: JFETs or MOSFETs can work up to 15 MHz as variable resistors but the circuits are tricky. For linearity of the control signal, two parts are used with feedback coming off the second to linearize the control signal.A Gilber cell analog multiplier using bipolar transistors will of course work.The various operational transconductance amplifiers are probably the easiest way to get electronic gain control. Check out the LT1228.Quote from: Shay on June 08, 2024, 11:04:55 pmSingle turn potentiometers are good to 15 MHz with proper layout.

author: HalFoster
author_rank: Frequent Contributor
timestamp: June 09, 2024, 10:10:52 am
content: Any reason not to use a pre-built attenuator? Pretty common on ePay and not very expensive - under $50 USD is fairly easy to find.Hal

author: Shay
author_rank: Regular Contributor
timestamp: June 14, 2024, 08:45:40 pm
content: Thanks for your insights. Single-turn 5k pots had much less signal distortion than 10-turn 5k pots, making the choice easy.

author: Terry Bites
author_rank: Super Contributor
timestamp: June 17, 2024, 06:22:26 pm
content: Pots are death to HF. So yeah, I wouldn't.I've been tinkering with THS7001 and 7002s lately, very handy preamp included on chip.Those AD amps have poor specs wrt to offeset voltage and bias currents.They are typical of wideband amps and they are designed for low Z drive.Barrie Gilbert, what a guy!

author: Kleinstein
author_rank: Super Contributor
timestamp: June 18, 2024, 05:48:44 am
content: Most signal generators have a fine adjustment for the amplitude. For coase gain steps (e.g. 5 dB steps) one can use switches / relays.

author: David Hess
author_rank: Super Contributor
timestamp: June 18, 2024, 11:02:59 am
content: Quote from: Terry Bites on June 17, 2024, 06:22:26 pmAnalog function generators used potentiometers for adjustment up to 40 MHz without problems. A JFET or MOSFET used as a resistor is good up to at least 100 MHz. Bipolar transistor Gilbert cells are good to 1 GHz.