Re: Fast switching current mirrors

From: John Larkin (jjlarkin_at_highlandSNIPtechTHISnologyPLEASE.com)
Date: 06/27/04 

Date: Sun, 27 Jun 2004 16:40:31 -0700

On 27 Jun 2004 13:31:55 -0700, saimazoom@hotmail.com (David Moreno)
wrote:

>Hi !
>
>> > Yes, but when you do that the switching element injects a charge
>> >in the capacitor that discharge it parcially. Serial switching is not
>> >really recomendable.
>> >
>>
>> I've done this a bunch of times. Start with a precise, slow current
>> source, maybe a high-beta PNP transistor inside an opamp feedback
>> loop. That dumps current into a PNP differential pair which steers the
>> current into your capacitor or dumps it to ground; wiggle the diff
>> pair bases to steer; I'm usually using differential ECL here. For even
>> more fun, use the "dumped" current to turn on a transistor that
>> discharges the cap when the current source is switched "off". Charge
>> injection is minimal.
>
> That's exactly the next circuit that I'm goind to work into. I'll put
>two series resistor to excite the base of the second transistor, and
>introduce
>some square voltage signal (arount 1Vpp) in the base of first
>transistor.
>
> I've simulated this topology and It seems to work well, but when you
>raise the frecuency the circuit seems to no respond well (too slow).
>
>> You need high-beta transistors to avoid base current:temperature
>> errors. Microwave PNPs are tempting, but tend to have low betas and
>> like to oscillate in this setup. BCX71K is a nice part, medium-fast
>> and high beta. I've done a nice 13 ns linear ramp this way, with a few
>> additional tricks.
>
> That transistor is too slow for my purposes (around 100MHZ) but I'll
>give
>it a Try with BFT93. Thanks for the Help ! =D
>
>> What are you building? How fast?
>
> I'm currently working in Hardware Neural Networks. I'm using this
>current
>mirror to store contributions in a capacitor (as a local memory). I've
>a working circuit at 1MHz or so, but now I'm trying to increase the
>working frecuency.
>
>Un saludo.

Oh, this box

http://www.signalrecovery.com/9650a.htm

uses differential PNP current steering to create interruptable linear
ramps; it's based on Pepper's fiendishly clever (but fatally flawed)
delay vernier patent. I think the clock period here is 80 or maybe 120
MHz, and they start/stop ramps on clock edges, with picosecond
precision.

John

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