Re: Fast switching current mirrors

From: Kevin Aylward (kevin.aylwardEXTRACT_at_anasoft.co.uk)
Date: 06/26/04 

Date: Sat, 26 Jun 2004 07:33:07 +0100

David Moreno wrote:
> Hi!
>
>>> and the
>>> frecuency of the switching is well below that.
>>
>> The ft is the frequency that the current gain is unity. The switching
>> speed of a transiser can be orders below this. Ft is only a guide.
>
> Ok, Which parameter do you think is most important then for a
> switching current ?

The Ft is good guide. There is no one parameter. Everything is a trade
off.

>
>> You havent posted the actuall circuit (LTSpice?), but from the text
>> it
>
> Yes, LTSPICE. By post do you mean in binaries, no ?

No, I meant the LTSpice full text schematic file.

>
>> looks like your switching the base. I am not going to wade through a
>> text schematic foir a better evaluation.
>
> Yep, sorry U_U. Next time I'll post the file.
>
>> The fastest way to switch (usually) is to current steer the *output*
>> current, not the base current. That is, keep the mirror always on and
>> divert the output from the required output to a dummy output. A mos
>> is a reasonable way to do this.
>
> Yes, but when you do that the switching element injects a charge
> in the capacitor that discharge it parcially.

Yes.

> Serial switching is not
> really recommendable.

Of course it is. It is a very useful technique and used a lot, e.g.
current steered DACs. Whether or not there is significant charge
injection depends on the particular application. I have certainly done
this in a charge pump for a PLL where this method was the only one that
achieved the required speed and accuracy.

*Whenever* you do a switch, there will be always be some level of charge
injection. For example, if you switch the base there will be feedthrough
via Cbc.

Best Regards,

Kevin Aylward
salesEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

Quantcast