Increase amplitude  range of a function generator - Page 1
Forum section: electronics / projects

author: gkmaia
author_rank: Frequent Contributor
timestamp: March 12, 2020, 01:43:53 am
content: I am trying to figure out what is the best way to increase the amplitude range of an HP 3311 function generator.The output amplifier ranges between -18/+18v based on it's supply.If I were to increase it's range 10 fold to -180/+180 what would be the best approach? Modify the circuit? A step up high frequency transformer on the output?

author: gbaddeley
author_rank: Regular Contributor
timestamp: March 12, 2020, 02:31:34 am
content: Use an external amplifier that meets slew rate, frequency range, and output current requirements.A transformer may work under light load conditions, because it also increases the source impedance by square of turns ratio. Sine may be OK, square & triangle will be distorted.

author: bitseeker
author_rank: Super Contributor
timestamp: March 12, 2020, 04:25:32 am
content: I second the external amplifier recommendation.

author: duak
author_rank: Super Contributor
timestamp: March 12, 2020, 09:54:16 pm
content: I believe the hp 3311A has a maximum frequency of 1 MHz. There's a discussion in this thread on an amplifier with similar voltage & frequency chc's:https://www.eevblog.com/forum/projects/analog-challengehigh-powerhigh-speed-amplifier-for-physics-experiment/Its output current is probably higher than what is wanted but it's easier to decrease rather than increase it.Cheers,

author: nfmax
author_rank: Super Contributor
timestamp: March 13, 2020, 08:13:14 am
content: The 3311A output stage is designed for a 600 ohm load, not the usual 50 ohm. I believe it will swing +/- 10V into this load. Stepping up to +/- 100V using a transformer would only work (well) for a load of 60 kohm or more. This may be OK for you, although finding a suitable transformer may be difficult.Edit: it looks like its only +/-5V into 600 ohms

author: gkmaia
author_rank: Frequent Contributor
timestamp: March 13, 2020, 06:33:36 pm
content: I am more inclined to go for the external amplifier.I did look at Duak link but I feel that is too complex. i am looking for a much simpler solution... like a very simple single transistor common emitter amplifier. What you think?

author: magic
author_rank: Super Contributor
timestamp: March 13, 2020, 07:11:17 pm
content: What bandwidth, what load impedance? How much offset voltage, nonlinearity, noise is acceptable?

author: gkmaia
author_rank: Frequent Contributor
timestamp: March 13, 2020, 07:28:26 pm
content: I will try to answer your questions to best of my knowledge. I am a hobbyist so I am not versed in some of these concepts.Amplitude amplification 10x the original. So from 18vpk to 180vpk.Up to 100khz max.Preffer 50ohm, but could stay with 600ohm as per standard for the device.No offset voltage.Linear.Some noise is acceptable.

author: Vovk_Z
author_rank: Super Contributor
timestamp: March 13, 2020, 08:24:25 pm
content: If you need 90 V p-p then something like LM3886 will do. But 180 V p-p and 100 kHz is not as easy to achieve.

author: duak
author_rank: Super Contributor
timestamp: March 13, 2020, 09:32:34 pm
content: Gkmaia, just some back of the envelope calculations:- 180 Vp into 50 ohms is 3.6 A- 180 V x 3.6 A is 648 W- 180 Vp is 127.3 VRMS for a sine wave- a linear amplifier's output devices dissipate the greatest power into a resistive load when the DC output voltage is half of the power supply rail voltage, in this case at least 180 V / 2 or 90 V. The dissipation at this point is 90 V x 1.8 A = 162 W. If the load is reactive (inductive or capacitive) or less than 50 ohms, the dissipation will be higher.An amplifier with these chc's is indeed possible, but it's a non-trivial bit of engineering. It can be simplified if the current and thus power can be reduced. Is there a particular load you'd like to drive? Also, is the output impedance there for back matching or does it have to drive a particular impedance orCan you accept a balanced ungrounded output? If so, then Vovk_Z's suggestion to use LM3886 could do, if two were bridged.About 20 years ago I needed +/- 90 V, 100 KHz amplifiers and made up a few using some hybrids from:https://www.apexanalog.com/I recall they were in the low USD 100 range but it was faster and cheaper to use them than to design, layout and build some from discretes.

author: magic
author_rank: Super Contributor
timestamp: March 14, 2020, 10:16:16 pm
content: Quote from: gkmaia on March 13, 2020, 07:28:26 pmThese are basically specs of a high power speaker amplifier. Somewhat more voltage, somewhat less current.Single transistor: not really, because such amplifier is inverting, AC coupled, has some 100V of DC offset and its gain depends on load impedance. With a few more transistors some of that could perhaps be overcome, particularly if AC coupling is acceptable. If accurate amplification of DC is required than I think there is no escape from the standard opamp/power amp topologies with LTP input stage and feedback. In any case, a moderately sophisticated class AB output stage will be required to avoid burning hundreds of watts of power in class A.

author: gkmaia
author_rank: Frequent Contributor
timestamp: March 14, 2020, 11:06:35 pm
content: Quote from: magic on March 14, 2020, 10:16:16 pmYes, that is exactly what I am looking for.

author: David Hess
author_rank: Super Contributor
timestamp: March 15, 2020, 12:59:08 am
content: An external amplifier is feasible but not trivial to design (or build at this power level) and as duak points out, the power requirements to drive a 50 ohm load are considerable.I would probably end up doing a "best effort" design starting with suitable output transistors and working back toward the input but the power requirements add considerable difficulty.