# How much drive current is needed?

Discussion in 'General Electronics Discussion' started by eefocus, Sep 6, 2012.

1. ### eefocus

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Sep 6, 2012
Hello everyone,I am working on a project of a transmitter.The major part is to generate short pulses.The only source is an oscillator.

However,to choose the proper oscillator,I need to know how much current is needed for the downstream circuit.So I simulate it in Agilent's ADS using transient simulation just like Spice.

The results show that different output impedances of oscillator lead to different currents in circuit. For a given output impedance,I wonder if the drive current can be lower and still make circuit work.Because many practical oscillator does not have so big drive current(for 20ohm,it needs 70mA).

So my question is how to get the drive current that is needed.Is that exactly what Spice show you?How can I know whether a device with a less drive current or power(voltage remains the same) can still work or not?

I still do not know how to limit the max current of circuit or power of source in Spice.Is there power source?

Thanks!

2. ### Harald KappModeratorModerator

11,813
2,753
Nov 17, 2011
An oscillator typically delivers only a very small amount of power. You will have to add an amplifier if yoiu need to draw serious power.

You can adjust the current to the load by changing the voltage. I=V/R is the key.

If you need to limit the power form a source in a SPICE simulation you have to model a "real" source, not an ideal one as SPICE does. This is easy:
- add a series resistor to an ideal voltage source or
- add a parallel resistor to an ideal current source

3. ### eefocus

4
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Sep 6, 2012
Thanks a lot for your help.
But for a practial oscillator,its power changes as the output current changes but output voltage remains the same.If I just decrease the voltage,downstram circuit will not work.Do you know how to just limit current max value but voltage remains the same?

Besides,my project's highlight is low power consumption.So I do no want to add another DC voltage or power amplifier.

4. ### Harald KappModeratorModerator

11,813
2,753
Nov 17, 2011
The current follows from the voltage and the load resistance: I=V/R. If you need to keep the voltage fixed, you have to increase the load resistance. Otherwise, if you limit the current, the voltage will decrease.´
That's the law (Ohm's law) - no chance any ploitician or engineer can change that.