Adding a power stage to a signal generator - Page 1
Forum section: electronics / beginners

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: November 08, 2016, 01:29:49 pm
content: I have a sine wave generator 1Hz - 1MHz frequency range and also has a amplitude knob however its output current is low probably a few 10's of mA. In some cases I need a stronger drive maybe upto 1Amp.I am looking at something simple probably a few transistors (discrete way) or a single chip solution (I would have to wait longer for this since the chip would have to be ordered). If the 1Hz to 1Mhz is too complex a drive stage I can settle for 1Hz to 20Khz for the moment it would be fine.Is an audio output amp the way to go? Any other suggestions?TIA

author: Kleinstein
author_rank: Super Contributor
timestamp: November 08, 2016, 01:40:04 pm
content: An audio amp is the right idea, at least for the lower frequencies.For the high frequencies (e.g. > 200 kHz) one may need to add something like an output resistor, as is get hard to impossible to make an amplifier with zero output impedance for these higher frequencies that is tolerating variable external loads. Even audio amplifiers and similar circuits have a noticeable ouput impedance at higher frequencies - quite often this is inductive like some 10 µH. YOu might need to add some more to make the amplifier more tolerant to capacitive loads.The normal output impedance is 50 Ohms as this matches the normal coax cables. A lower value (like 10 Ohms) might be possible.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: November 08, 2016, 02:03:55 pm
content: Thanks Blueskull and Klienstien,Can I cook up something quick I mean I have some transistors or if mosfets would do the trick I can go in for that.Also I forgot about the power supplies would single + supply do the trick or would I need dual + - power supplies?I found a single supply amp based on the LM386 herehttp://www.bristolwatch.com/radio/lm386_power_amp.htm

author: pigrew
author_rank: Frequent Contributor
timestamp: November 08, 2016, 02:14:07 pm
content: Quote from: ZeroResistance on November 08, 2016, 02:03:55 pmIf you don't care so much about distortion, you could build a simple push pull output stage with BJTs (a few BJTs would work). If you need low distortion, you'd need to design a feedback amplifier (much more complicated, a dozen or so BJTs).If you don't mind filtering out DC, then you can use only a positive supply. If you have to preserve DC, and your output has to be able to go negative, then positive and negative supplies would be needed.Sent from my LG-D850 using Tapatalk

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: November 08, 2016, 02:23:39 pm
content: Quote from: pigrew on November 08, 2016, 02:14:07 pmYes I don't mind distortion and filtering out DC, generally the audio amps are rating in output power (W).For eg. the LM358 circuit I posted above is rated for several watts by using TIP31 and TIP32.Lets say the wattage is 2 watts output power.Do they typically rate it for 8 ohm load and if so would it be right to use this equation to get the current output in the load?W = I2R;soI2= W / RI = sqrt(W/R) = 0.5 AmpI need at least 1 Amp so would it be right to say that I need at leas a 4W output stage audio amp?

author: Kalvin
author_rank: Super Contributor
timestamp: November 08, 2016, 02:26:25 pm
content: Power Op Amp, Max. 5V, Output Signal Swings Within 200mV of Rails at 2A Output Currenthttp://www.ti.com/product/OPA5691.5A, 24V, 17MHz, Power Operational Amplifierhttp://www.ti.com/product/opa564

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: November 08, 2016, 02:48:19 pm
content: Quote from: Kalvin on November 08, 2016, 02:26:25 pmThanks Kalvin,Nice find! Yes the power op amp looks good but it is 5V supply and going at $4 for 1K pcs and yes TI has a range of other power amps which operate on higher voltages > 15V .I'd like my output signal amplitude to be a bit higher probable around 10Vp.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: November 08, 2016, 02:53:59 pm
content: Quote from: blueskull on November 08, 2016, 02:50:20 pmIn the short term that is for immediate use I would go in either for a discrete version or buy a popular audio amp from a local source. For long term use I could go in for specialized chips mentioned by kalvin et alThanks.

author: Kalvin
author_rank: Super Contributor
timestamp: November 08, 2016, 02:57:30 pm
content: Quote from: ZeroResistance on November 08, 2016, 02:48:19 pmIf you read carefully my post, the opa564 will go up to 24V and will have better GBW.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: November 08, 2016, 03:30:53 pm
content: Quote from: Kalvin on November 08, 2016, 02:57:30 pmYes I did read it's a real good device better specs and good price!

author: CJay
author_rank: Super Contributor
timestamp: November 08, 2016, 03:39:31 pm
content: Quote from: ZeroResistance on November 08, 2016, 01:29:49 pmhttp://uk.rs-online.com/web/p/operational-amplifiers/0301599/

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: November 08, 2016, 04:03:42 pm
content: Quote from: CJay on November 08, 2016, 03:39:31 pmAlas! the product has been discontinued.

author: CJay
author_rank: Super Contributor
timestamp: November 08, 2016, 06:00:43 pm
content: Ach, damnit so it has.I'll have to find an equivalent for myself as well now.Have you looked at 'simple' amplifier chips like the TDA2030 etc?

author: Kleinstein
author_rank: Super Contributor
timestamp: November 08, 2016, 06:31:38 pm
content: The TDA2030 is cheap, but also not that fast and not known for very good stability with a more difficult load.I would ad least consider a more modern audio amplifier. They tend to be faster and less critical on the load. Likely you still need the often seen RL pair at the output to make it stable with any load. I would consider something like an LM1876.Keep in mind the audio amps need quite some amplification to be stable - maybe even more with a critical load.A good fast OP would be the LT1210, though rather fast for just 1 MHz max. Though a square wave also include quite some overtones.

author: David Hess
author_rank: Super Contributor
timestamp: November 08, 2016, 07:10:50 pm
content: Audio amplifier designs are generally going to be too slow although one of my first designs was good to almost 1 MHz.Check outLinear Technology application note 18for some ideas. Look up "diamond buffer" which is a higher performance 4 or more transistor implementation of what ZeroResistance posted.Quote from: Kleinstein on November 08, 2016, 06:31:38 pmThat was the first thing I thought of and almost perfect for this job.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: November 08, 2016, 07:36:02 pm
content: thanks CJay, Kleinstein and David for your responseSo I have got power opamps, audio amps, diamond buffer ... and more to look into ..Thanks again!