is it varying Emitter - collector resistance according with base volt ?

Experts is it varying Emitter - collector resistance according with base volt ?
( it may a simple question but i ma totaly confused please replay ?)

I want to varying Emitter - Collector Resistance in accordance with base to emitter Volt
Like a digital Pot

YES, you can use a scope as a curve tracer to measure the current gain in a transistor or the voltage gain in a FET or the transconductance of a MOSFET.
Simply use series resistor on emitter to monitor current and display as Channel 1 and Voltage as Channel 2 and use scope in XY mode to see the input versus output.

Remember transistors are current amplifiers but are also non-linear with voltage, current, temperature. A current source ideally is independent of voltage but leakage in CE will make that have a VI slope on the Collector emitter response curve.

So if you want to generate a base current, use a high enough resistance from a voltage source minus Vbe offset to estimate current, so your voltage source is approximately a current source. "Curve tracers" use actual current sources instead for more accuracy. But this simple approach will give you the idea. The load resistance on the output or "Load line" slope is used to determine the operating point on the VI output response. rather than Vin vs I out, use Vout-Iout for this.. But to see gain.. use I out over I in to get hFE or Beta or current gain ( synonyms)

in the example above with the XY mode of scope . use a unipolar signal to sweep the input voltage . any shape sine or triangle 50 to 200Hz will do say 100mV p-p biased at 0.65V on the base thru say a 1K resistor for a starting point.. Observe maximum base current and collector current limits with suitable collector resistor for voltage applied.

added: using 1K base resistor with 1mA input would require 1.65V approx zero to peak voltage on the generator. Zero current would be < 0.6V on Vbe. so your generator should set with aDC offset and signal going from 0.55 say to 1.65 to generate 0 to 1mA input.. Then carefully adjust gain and offset so Vbe never goes negative. to increase base current. More sophisticated sweep source can be made with a current source by design, but for starters, that will allow you see the characteristics.Then using a decade resistor box for collector can change the load line curve. Emitter current sensor can be chosen for range desired like 10 Ω would be 10mV/mA for X axis current. Then adjust scope horizontal gain with that channel gain and offset. for NPN set the null point to the bottom left corner. for PNP set to upper left corner which is inverted polarity.

A bipolar transistor can work as variable resistor in saturation region of it's characteristic, e.g. up to 100 mV or 200 mV Vce. Above this voltage, it's more and more acting as a current source.

https://en.wikipedia.org/wiki/File:Current-Voltage_relationship_of_BJT.png

I read plot instead of POT (doh)

.. yes I think you want an "Active Load" using a transistor.

Since Vin to I out is non-linear due to Vbe and base resistor, the scope method may help you visualize the characteristics.

This is an excellent way to test batteries and SMPS for step active load response.

But as a POT it will be more logarithmic rather than linear due to the Vbe & hFE response and hence generate a lot of distortion for large signals. Small signals may be ok but limited dynamic range. They do make digital programmable POTs designed for such occasions and are fairly cheap.

can i measure emitter- collector Resistance with the help of a mulitmeter because I havent a CRO ?

You should consider that the output I/V characteristic is basically non-linear, so you'll get slightly different values for different voltages even in the 100 mV Vce range,

You can of course measure a transistor characteristic with a multimeter, better use separate instruments for Ic, Vce, Ib. Variable voltage sources are needed in addition, or at least variable resistors. Supplying constant Vbe won't work well due to the steep characteristic, you better use constant Ib.

so i cant use that . is FET is usable the source to drain resistance varying with gate voltage ?

so i cant use that

Click to expand...

You didn't tell yet a specification including voltage, expected linearity.

FETs have a larger linear range as variable resistors, but they are also far from being ideal. You can review previous threads related to this topic.

the link is not working

YOu can get a 150MHz digital scope now for< $250. YOu need one to learn. If you know already how they work , they do make DMM's wit transistor sockets for measure hFE but not load resistance. But witha simple breadboard. you can design an active load ( see my link ) and test the breadboard with any DMM

the link is not working

Click to expand...

Fortunately edaboard has a search option. Did you try "FET variable resistor"?
Or append html to the link.