How to control a constant current source?

[Fox zhou]

Dear all,

I'd like to charge a capacitor with a constant current source in short
period of time(1ns-10ns). At the end of the time, I want to stop the
charging. I propose to use a switch to control the constant current
source. Turn on at the start and turn off at the end.

But where can I find the corresponding switch? (1GHz)
Does any one know there are the controllable constant current source
chips can complete this task?

--Fox

 

[yigiter]

you can use a rgulator for constant current source. for example in 7805
datasheet, you can find a sample circuit. for switching, i have not got
much idea but maybe you can use a microcontroller to adjust the time.

 

[Rich Grise]

you can use a rgulator for constant current source. for example in 7805
datasheet, you can find a sample circuit. for switching, i have not got
much idea but maybe you can use a microcontroller to adjust the time.

Yeah, there ya go.

Submit a PIC circuit with a 7805 that outputs a 10 nsec ramp with
1 nsec resoolution, and switching times of 1 psec.

OK.

It won't be from me.

Good Luck!
Rich

 

[John Larkin]

Dear all,

I'd like to charge a capacitor with a constant current source in short
period of time(1ns-10ns). At the end of the time, I want to stop the
charging. I propose to use a switch to control the constant current
source. Turn on at the start and turn off at the end.

But where can I find the corresponding switch? (1GHz)
Does any one know there are the controllable constant current source
chips can complete this task?

--Fox

The usual way to do this is a PNP differential pair steering a source
current pumped into the emitters. A typical timing ramp might be
10-100 ns, 5-10 mA, maybe a few volts of swing, as much as the
downstream amps or comparators can stand. The current source can be an
active source (another PNP inside a feedback loop) or just a resistor
to a highish voltage if extreme accuracy isn't needed. BXC71's work
well; Ft isn't a big issue here if you drive the bases hard. You can
use faster PNPs, like the NEC parts, but they tend to oscillate.

A ramp cap can be reset by an open-drain CMOS part, a regular CMOS and
a schottky diode, or by a gaasfet. Keep in mind that every
semiconductor on the charging node is a nonlinear capacitor. 2%
linearity is easy, 0.1% requires serious technique and tricks.

A really fast ramp can use a very fast current source gated by two
schottky diodes operating as a current divertor. You can get packaged
dual schottkies down to about 0.25 pF. Careful here, schottkies are
leaky. Really fast ramps (say, 10 ns or less total ramp time) are
tricky, as parasitics will cause ringing and the ramp will be ugly.


John

 

[John Larkin]

The usual way to do this is a PNP differential pair steering a source
current pumped into the emitters. A typical timing ramp might be
10-100 ns, 5-10 mA, maybe a few volts of swing, as much as the
downstream amps or comparators can stand. The current source can be an
active source (another PNP inside a feedback loop) or just a resistor
to a highish voltage if extreme accuracy isn't needed. BXC71's work
well; Ft isn't a big issue here if you drive the bases hard. You can
use faster PNPs, like the NEC parts, but they tend to oscillate.

oops, BCX71.

John

 

[Bill Sloman]

Dear all,

I'd like to charge a capacitor with a constant current source in short
period of time(1ns-10ns). At the end of the time, I want to stop the
charging. I propose to use a switch to control the constant current
source. Turn on at the start and turn off at the end.

But where can I find the corresponding switch? (1GHz)
Does any one know there are the controllable constant current source
chips can complete this task?

The usual way of doing this is to feed the constant current into the
common emitter of a long-tailed pair, and put the capacitor in series
with the collector of one of pair. You can discharge the capacitor
after you've measure the voltage with some kind of switch - I've used
FETs, MOSFETs and a second long-tailed pair of complementary
transistors in various different examples.

For this sort of speed you want to build the fast-switching
long-tailed pair with RF bipolar transistors - when I was doing this
back in the 1990s, we mostly used NPN BFR92 parts, switching a roughly
10mA current.

These are 5GHz parts. Farnell now list NPN 10GHz parts for the same
sort of price. Farnell list two 5GHz PNP parts - the BFT92 and the
BFT93 - which we found handy as fast recharge sources.

The controllable constant current source for the tail is easy enough -
we tended to use LM317 three-terminal volage regulators as the basic
current sources (see that data sheet for the circuit) but they look
capacitative at high frequencies, so you need the highest possible
resistance in series to stop this driving the the transistors of the
long-tailed pair into oscillation.

You can also use "constant current diodes" - FETs with the gate tied
to source - for lowish currents (5mA or lower), but they are just as
capacitative.

 

[nyffeler]

Dear all,

I'd like to charge a capacitor with a constant current source in short
period of time(1ns-10ns). At the end of the time, I want to stop the
charging. I propose to use a switch to control the constant current
source. Turn on at the start and turn off at the end.

But where can I find the corresponding switch? (1GHz)
Does any one know there are the controllable constant current source
chips can complete this task?

Are you looking for a time to amplitude converter?
If so, have a look at
http://ej.iop.org/links/q24/R3eIq,maJdzdv1f2yjwktw/e003n2.pdf
or a commercial one on
http://www.canberra.com/products/692.asp

 

[Fox zhou]

Are you looking for a time to amplitude converter?

If so, have a look at
http://ej.iop.org/links/q24/R3eIq,maJdzdv1f2yjwktw/e003n2.pdf
or a commercial one on
http://www.canberra.com/products/692.asp

You are correct. I want to hunt a circuit and integrate on my board.
Could you give me some tips or tell me the circuit scheme or where I
can find circuit details? Thank you.

--Fox

 

[Oliver Betz]

[email protected] (nyffeler) wrote:

[...]

Are you looking for a time to amplitude converter?
If so, have a look at
http://ej.iop.org/links/q24/R3eIq,maJdzdv1f2yjwktw/e003n2.pdf

is this a URL with session ID? Or only accessible for registered
users? It doesn't work here.

Any other way to get the article?

TIA,

Oliver