trans.curve.tracer

7/28/2019 Trans.curve.tracer

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Practical No. 13

Object:To Design a Transistor Curve Tracer System Using an Oscilloscope

Apparatus:

Function generator

Oscilloscope

Dual Power Supply

Components:

IC Binary Counter (74LS93N)

IC OP-Amplifier (LM324N)

NPN Small Signal Transistor (2N3904)

Resistors

Jumpers

Theory:Curve Tracer:

Curve tracer is an instrument used to plot the Current-VoltageCharacteristic curves of two or three terminal devices.Curve tracers are particularly used to observe the characteristic curves of

transistors are called Transistor Curve Tracers.

Need of a Curve Tracer:

The ninety percent of datasheets do not have characteristic curves. The

remaining ten percent will contain characteristic curves for a transistorselected randomly among a batch of similar transistors.

No two transistors are exactly the same. This means that the characteristic

curves given in the datasheet wont apply to your transistor although theyboth will have the same part number as manufacturer.

We can plot our AC load line and select a proper bias point called

Quiescent Point for Small-Signal approximation.


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Operation:Binary Counter:

The binary counter 74LS93 is used here in mod -16 mode in whichthe counter will count from 0000 to 1111 (16 combinations). This is done byapplying clock signal to Clock A. QA is connected back to Clock B. R01 and R02are grounded so that manual resetting is ineffective. After count of 16 thecounter will reset automatically.

The Weighted Summer Amplifier:

The binary counter has 4-bit output. Each of these four outputs areconnected to the inputs of a weighted summer amplifier. In this amplifier we haveset each of the four gains to be 51 , the reason for which is simply to convert 5VTTL to 1V signals. The output is inverted and is the sum of the effects of 4-bitsinputs. For example, when count is 0000, the output will be 0+0+0+0=0V, for0001, the output will be 0+0+0+1=1V. Consider the case when count ismaximum that is 1111, the output will be 1+1+1+1=4V. Had the gain not be 51 ,say it was unity then output would have been 5+5+5+5=25V, a voltage that isquite large to be applied to a transistor base-emitter junction!

The Unity-Gain Inverting Amplifier (Polarity Reversal):

The output of the weighted summer is inverted (180 degree) out ofphase with respect to the inputs therefore, we have utilized another OP-Amplifier, but this time no gain rather polarity reversal is being done. In this waythe output will again be in phase with the input bits.

The Transistor under Observation:The output of the second OP-Amp is then fed to the base of the

transistor to be tested. The transistor will respond to the different voltagecombinations at base and draw corresponding collector currents. The output istaken across an emitter resistor through an oscilloscope.

The Oscilloscope Configuration:

The oscilloscope will be used here in X-Y mode. The channelcorresponding to horizontal axis has been connected to an AC source usingfunction generator whereas the channel corresponding to vertical axis has been

connected to transistor output (across emitter resistor).There is reason behind the use of AC supply to power the transistor. If a DCsource was used to power the transistor then at the output of oscilloscope thehorizontal(x-ordinate) of the x-y point will remain fixed and the moving x-y pointwill not be obtained, and if this happens we will not be able to obtain the curvesbecause the curves are obtained due to the very fast motion of x-y point at thedisplay!.


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Problems with Commercial Curve Tracers:

They do look like oscilloscope but you cant use them like oscilloscopes.

Usually they are sensitive to handle.

Too much costly that they are hard to afford.

Advantages of Our Design: Our curve tracer is a mixed circuit that is analog and digital.

It will convert any existing dual-channel oscilloscope that supports x-y

mode into a curve tracer.

Little configurations are needed for the oscilloscope to be used for curve-tracing function.

The oscilloscope can be used both as a curve tracer and to display wave-

forms.

Design is simple, effective, utilizes fewer components, less power hungry,

and yet at a very cheap price.

Circuit Schematic:


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The Schematic Diagram of Transistor Curve Tracer.


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PCB BOARD:

Top View of Assembled Circuit

Expert View of Assembled Circuit


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P.C.B. Layouts:

Universal View of PCB Layout

Routed Bottom of PCB Layout


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Simulations and Real World Results:The Simulated Result on Multisim

This is the simulated result on the simulating software.

The Real World Result.

This result is obtained on the screen of Oscilloscope.

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