I recall once seeing a fully solid state circuit that uses capacitors, resistors and a transistor to "mimic" the effects of an inductor. I can't seem to find it now. Can anyone point me in the right direction?
Simulated inductance that is grounded on one end is easiest, I think.
-- John Popelish
Google for "gyrator"
See the various Gyrator-based filters on the S.E.D/Schematics page of my website.
...Jim Thompson
-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |
Why would anyone bother? Is there some obscure advantage with it over conventional coils?
-- "What is now proved was once only imagin\'d." - William Blake, 1793.
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A conventional hundred henry coil weighs several pounds, is bigger than the printed circuit board, and has a terrible Q.
I read in sci.electronics.design that Reg Edwards wrote (in ) about 'Solid State Inductance', on Thu, 6 Jan 2005:
Get some decent Mumetal cores. 10kH in about a 2 inch cube, with a laminated core. More with a toroid. Q indeed was not promising at 1 kHz, but this operated at 10 to 20 kHz.
-- Regards, John Woodgate, OOO - Own Opinions Only. The good news is that nothing is compulsory. The bad news is that everything is prohibited. http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
+9V o----------o | | .-.220k | | |R3 | | | | '-' | | | C1 | | 100u 1uF | Jelly Bean NPN - + | | || || | |/ A o---||-----o----||-----o--------| (BC546) || | || | |>
CC | | | | ___ | | '--|___|--- | ---------o 100 | | R1 | | B o .-.220k .-.1k | | |R4 | | | | | | | | '-' '-' | | | =o= =o= =o= GND GND GND
Capacitor CC is just for decoupling. Inductance appears across points A and B Henries approx' (R3 in parr' with R4) x C1 x R1 This example gives L = 10H R1 controls the 'Q' value
As you can see the circuit is very similar to a simple emitter follower but adding just R1 makes a vast difference electrically . It's more usual to use a opamp follower in place of the transistor as it can give better performance. There was last Feb', a long thread here from Terry Pinnell, ("Coupling capacitor voltage rating") demonstrating that this kind of gyrated inductance can inadvertantly appear without even being asked for.
(created by AACircuit v1.28 beta 10/06/04
isn't copper solid state? Everytime I see this tread that pops into my mind. Pat
It's much more immune to external signals induced by external varying magnetic fields. Traditional coils may require mu-metal shielding to get near as much immunity, which usually costs a lot of money, and there's nothing obscure about money. There are probably a few more obscure things such as hysteresis, core saturation and inducing signals into nearby coils that an "electronic inductor" doesn't do. OTOH, real inductors don't need a separate DC power supply to have inductance. Like most anything in engineering, there are many trade-offs, and which one is best depends on the application. There are still plenty of applications where a "real" inductor is the best or only solution.
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To understand the concept you have to go back to the '60's where every new product that used transistors instead of tubes had the words "Solid State" proudly displayed, often in a fancy cursive italic font, on the front panel. Just using copper wasn't enough.
OTOH, you can make a gyrator with a vacuum tube as the active element...
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One reason is size... You can build a massive inductor with a gyrator, in a volume that is tiny compared to the equivalent real inductor. Some years ago, I designed the first modem, to use this design to handle the internal inductor needed. The key was in this case, that the inductor at that time could not be fitted into the low profile wanted, without taking an inordinate amount of board space. At the time the people handling the approval testing were amazed at how well it performed, and just how small the line interface could be made. Most modern modems now do this.
Best Wishes
The name of the circuit to make an equ inductances is called a "Gyrator".
A study was done by Roger Rerden (spelling) on Gyrators.
One advantange is to replace inductors in LC filter designs.
A tranistor common emitter circuit with the capacitor across the base emitter will simulate an inductor at the tied collector end.
As freq goes up the cap starves the base current turning the transistor more off.
This is obvious as the common emitter inverts signals (180deg) phase shift.
JG
direction?
Damn I forgot the smiley! Pat
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