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Surjan Dogran's Easy Peasy 70v peak-peak Opamp for <img
Surjan Dogran's Easy Peasy 70v peak-peak Opamp for <img
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Old 6th November 2017, 09:12 AM   #1
xrk971 is offline xrk971  United States
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Surjan Dogran's Easy Peasy 70v peak-peak Opamp for <img
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Member @mediatechnology (Wayne Kirkwood) told me about how to bootstrap ANY opamp using two TO-92 transistors and 4 resistors using Surjan Dogran's Opamp Bootstrap technique first published in Electronic Design, in 1974. I needed an instrumentation opamp that could swing 65v p-p but they don't make opamps that can do that. So I was using a modified Apex FX8 sans the output stage and with some mods to higher voltage transistors etc, it was actually working pretty well. Then I saw this article from 1974 and had to try it. I used a JRC 5534D opamp, normally rated for +/-22v rails. I followed this diagram and added qnty 4 x 10k resistors and a 2N5551 and 2N5401 BJT's to the circuit as shown. That's it.

Surjan Dogran's Easy Peasy 70v peak-peak Opamp for <img


I then boosted the rails to +/-44v (as 44/2=22v so within safe limit). Here is a photo of my opaamp. I have a couple of extra's features already there from earlier build, including: a Coilcraft 1.2uH inductor with 10R Thiele output network, a 680pF RF lowpass input filter, and a 36pF silver mica cap in parallel with the feedback cap:
Surjan Dogran's Easy Peasy 70v peak-peak Opamp for alt=-bootstrap-opamp-setup-jpg

And here is the scope shot - it looks fantastic and I am easily driving a clean 70v p-p output and it looks really nice. For 2.68v p-p input I a, getting 70v p-p output. I think that this has got to be perhaps the easiest and most cost-effective way to put your opamp on steroids. Of course, it is not meant for driving low impedance loads - but for cost probably less than $0.25 in parts added to my opamp circuit, I have a bonafide high-swingin' contender here. I bet the THD is now even lower than before:
Surjan Dogran's Easy Peasy 70v peak-peak Opamp for alt=-bootstrap-opamp-scope-jpg

Anyhow, this worked out really well and hope you all find it useful. Seems like it would be a great ultra-low distortion front end for a higher power output stage.
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File Type: jpg Bootstrap-opamp-setup.jpg (411.2 KB, 1491 views)
File Type: jpg Bootstrap-opamp-scope.jpg (353.0 KB, 838 views)
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Last edited by xrk971; 6th November 2017 at 09:16 AM.
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Old 6th November 2017, 11:08 AM   #2
ojg is offline ojg
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Surjan Dogran's Easy Peasy 70v peak-peak Opamp for <img
Thanks for sharing, it's a great idea.

Since the supply voltage across the opamp is constant, it should be ok and perhaps even beneficial to add a decoupling capacitor between V+ and V-.
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Old 6th November 2017, 11:27 AM   #3
xrk971 is offline xrk971  United States
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Surjan Dogran's Easy Peasy 70v peak-peak Opamp for <img
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Originally Posted by ojg View Post
Thanks for sharing, it's a great idea.

Since the supply voltage across the opamp is constant, it should be ok and perhaps even beneficial to add a decoupling capacitor between V+ and V-.
I actually have a 100nF X7R between the two rails right underneath the IC. You can also see I have two 100uF 50VA rail caps near he Vcc and Vee pins of the opamp. Wayne Kirkwood actually recommends a 470pF to 1nF film cap between base and collector of the bootstrap transistors. I did not use it and seems to work but probably a good idea for enhanced stability.
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Old 6th November 2017, 11:33 AM   #4
BesPav is offline BesPav  Russian Federation
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https://m.eet.com/media/1152270/24127-45890.pdf
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Old 6th November 2017, 11:46 AM   #5
xrk971 is offline xrk971  United States
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Surjan Dogran's Easy Peasy 70v peak-peak Opamp for <img
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Thank you! I see the extra steps needed to ensure stability now over operating range.

Quote:
To ensure that your design meets input common-mode range under all conditions, you must address dc condi- tions, transient conditions, phase rever- sal, and power-up conditions.
Good stuff!
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Old 6th November 2017, 12:03 PM   #6
DUG is offline DUG  Canada
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So, how is this OK with maximum input voltage?
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Old 6th November 2017, 12:04 PM   #7
Mooly is offline Mooly  United Kingdom
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Surjan Dogran's Easy Peasy 70v peak-peak Opamp for <img
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Originally Posted by xrk971 View Post
I needed an instrumentation opamp that could swing 65v p-p but they don't make opamps that can do that.
Have a look at the LTC6090 and LTC6090-5 which are 140 volt capable opamps.


Supply Range: 4.75V to 70V (140V)
n 0.1Hz to 10Hz Noise: 3.5μVP-P
n Input Bias Current: 50pA Maximum
n Low Offset Voltage: 1.25mV Maximum
n Low Offset Drift: 5μV/C Maximum
n CMRR: 130dB Minimum
n Rail-to-Rail Output Stage
n Output Sink and Source: 50mA
n 12MHz Gain Bandwidth Product
n 21V/μs Slew Rate
n 11nV/√Hz Noise Density
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Old 6th November 2017, 12:11 PM   #8
BesPav is offline BesPav  Russian Federation
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Quote:
Originally Posted by xrk971 View Post
Thank you! I see the extra steps needed to ensure stability now over operating range.

Yes, its better not to cheat and make things right - just add simple gain stage.
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Old 6th November 2017, 12:13 PM   #9
abraxalito is online now abraxalito  United Kingdom
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There are a few high voltage opamps from TI/BB for example OPA445, OPA452/3 and OPA454.
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Old 6th November 2017, 12:56 PM   #10
xrk971 is offline xrk971  United States
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Surjan Dogran's Easy Peasy 70v peak-peak Opamp for <img
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Originally Posted by Mooly View Post
Have a look at the LTC6090 and LTC6090-5 which are 140 volt capable opamps.


Supply Range: 4.75V to 70V (140V)
n 0.1Hz to 10Hz Noise: 3.5μVP-P
n Input Bias Current: 50pA Maximum
n Low Offset Voltage: 1.25mV Maximum
n Low Offset Drift: 5μV/C Maximum
n CMRR: 130dB Minimum
n Rail-to-Rail Output Stage
n Output Sink and Source: 50mA
n 12MHz Gain Bandwidth Product
n 21V/μs Slew Rate
n 11nV/√Hz Noise Density
Thanks, Mooly. I did not know about these. Will need to get a few to try. Not a bad price at $7.50 ea.

And thanks Abraxalito. Those Tl's look good too. Things have progressed quite abit since I last looked many years ago they were like $50. I guess all they had to do was add a couple of transistors and resistors and diodes internally.

It's still nice to take one of our favorite audio-grade opamps and add large voltage swingnront though.
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Last edited by xrk971; 6th November 2017 at 01:01 PM.
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