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Optimizing the 2N5458 JFET Lab


Semiconductor device experiment examining the 2N5458 n-channel general purpose JFET. Theoretical concepts discussed in the lecture course Analog and Semiconductor Devices (EE 211) and verified using laboratory experiments, hand calculations, and computer simulations.

Objectives:

1. To measure the transconductance, and static and dynamic drain resistances of a junction field effect transistor (JFET) at a specified Q‑point.

2. To measure the common‑source output (drain) characteristics and the transfer characteristic of a JFET.

3. To utilize PSPICE to generate common‑source output and transfer character­istic curves.

4. To match the PSPICE generated output and transfer characteristics to those of a specific transistor by adding/modifying model parameters in the PSPICE .MODEL statement.


Equipment and Materials:
1. Curve tracer
2.
2N5458 JFET, 2 each
3. Computer with PSPICE software



Procedures:

1. From curve tracer measurements, accurately record the output character­istics of two 2N5458 FETs. Sweep VDS from 0 to 20V, and set the step generator for VGS ≤0.5V per step. Draw the characteristics on at least half of a sheet of graph paper, and pay particular attention to the slope of the VGS curves in the beyond pinch off region.

2. Determine the static and dynamic drain resistances (RD and rd) for each of the two 2N5458 FETs at VGS = 0V, VDS = 15V. Compare these values to those shown in the 2N5458 specifications.

3. From curve tracer measurements, accurately record the transfer character­istics of the two FETs at VDS = 15V. Again, use at least half of a sheet of graph paper for each curve.


4. Determine the transconductance (gm) of each FET at VGS = 0V, VDS = 15V Compare the measured values to those shown in the 2N5458 specifications.

5. Repeat Procedures 2 through 4 for an operating Q‑point of ID = 1mA, VDS = 10V. Comparison with 2N5458 specifications is not required.

6. Write a PSPICE file to generate the output characteristics of a generic N-­Channel JFET (all parameters set to their default values).

7. Modify the .MODEL statement(s) to include values for VTO, BETA, and LAMBDA which allow the improved PSPICE generated output characteristics to match those of the real devices.

8. Write a PSPICE file to generate the transfer characteristic of a generic N‑Channel JET (all parameters set to their default values).


9. Modify the .MODEL statement(s) to include values for VTO, BETA, and LAMBDA which allow the improved PSPICE generated transfer characteristics to match those of the real devices.

10. Compare the final PSPICE generated characteristics to those measured on the curve tracer, by superimposing one on the other.

Procedure & Data:

1. In the given experiment the output characteristics of two 2N5458 JFET were examined using two specific techniques. First the devices were examined with the use of a curve tracer. To measure the voltage step rather then the current step generally associated with the Type 575 curve tracer a 1k Ohm resister was inserted from the base to emitter post. With the device properly connected in the curve tracer the VDS was swept from 0 to 20V and VGS ≤ 0.5V per step. The output family of curves were then examined and sketched for each device (figure 1.1 & 2.1).

Once an understanding of the output characteristics were plotted the static RD, and dynamic rd drain resistance were then approximated from the output plots. This was approached by picking a value on VGS = 0 in the operating range and determining the ID at that point (figure 1.1 & 2.1). RD and rd were then compared with the values given on the specification sheet for the 2N5458 JFET (table 1).

Table 1: Drain Resistance vs. Given Specifications for 2N5458 JFET's


Next the transfer curves were measured on the curve tracer and plotted. This was accomplished by setting the VGS = 0.5V/div. and ID = 0.5mA/div. and examining the drain current vs. gate source voltage. Once plots were drawn for each of the 2N5458 JFET's it was required to determine the transconductance (gm) for each of the devices. Setting VGS = 0V, VDS = 15V, calculating gin (E.1), and comparing the results to the given specifications the transconductance was determined (table 2).

gm = ∆ ∙ I / ∆ ∙ VGS (E.1)

Table 2: Transconductance (gm) for 2N5458 JFET's


After completion of the above steps it was required to repeat the procedure above at a given Q‑point where ID = 1mA, VDS = 10V. Upon examining RD, rd for graph 2.1 it was found that the Q‑point lied between two curves. Therefor by determining the slope of both the above and below curves and average the two curves resulted in the best approximation for the drain resistance for graph 2.1 (table 3). As for determining the transconductance (gm) for the given Q‑point was similar to the above method (E.1). The transconductance was calculated from the transfer curves using straight line approximation and the newly found data recorded (table 3).

Table 3: Drain Resistance & Transconductance (gm) for 2N5458 JFET's





2. In the second part of examining the output characteristics of the 2N5458 JFET's, Pspice was used to simulate the output response. To understand the general behavior of the family of curves in graphs 1.1 and 2.1, a Pspice generic N‑Channel NET simulation was ran (graph 3.1) and compared to the measured output taken from the Type 575 curve tracer. To better simulate the measured output the Model statement was used to define VTO, BETA, and LAMBDA. VTO or the pinch off voltage was determined by examining the measured output (graph 1.2, 2.2) from the transfer curves where the slope of the curve approaches zero (table 4). BETA was calculated (E.2), again using the data taken off the measured transfer curve (graph 1.2, 2.2) where IDSS was the maximum point of the curve.

BETA = IDSS / (VP)2 (E.2)

In determining LAMBDA (λ), the slope of the transfer curve needed to be approximated using straight line approximating from the measured transfer curve (graph 2.1, 2.2) over it's complete range. Therefor using equation (E.3) for determining the slope of the line, applying the equation for the slope intercept (E.4) and solving for the inverse of the x intercept (E.5) for VG produced the λ result required for the Pspice simulation (graph 3.2, 3.3).


Slope = 1 / rd (E.3)

y = m ∙ x + b (E.4)

x = 1 / λ (E.5)

Table 4: Characteristic Parameters for 2N5458 JFET's



Next transfer characteristics simulations were ran on Pspice and compared to the measured plots. First to gain a better understanding of the transfer characteristics a simulation was ran using only the default transfer settings (graph 4.1). As with before once an understanding of the default transfer curve was gained a more specific simulation using VTO, BETA, and LAMBDA (λ) were plotted (graph 4.2, 4.3) (table 4). In determining the specific parameters required for the plot the following equations and technique was used as above (E.3), (E.4), (E.5). Upon completion, the plots were then compared to graphs 1.2, 2.2 take from the Type 575 curve tracer.





Conclusion & Discoveries:

It was learned that in analysis of a NET device one must carefully 'pick' points off the family of curves, in the operating range to approximate the output characteristics of the device. Analysis of the device is not mathematically derived, yet it is rather experimentally derived.

Analysis of the transfer curve was accomplished by straight line approximation. One may ask the question if a mathematical equation can be derived to better approximate the curve?

Variations of Ohms law allowed for one to simply calculate the output characteristics of the devices.

In closing, the 2N5458 JFET device was examined closely with two specific techniques applied. It was discovered that the measurements taken off the curve tracer scope in comparison with the data simulations produced from Pspice agreed with in acceptable error bounds (±5%). Overall the characteristics of the 2N5458 NET devices were learned and a general understanding of the outputs will provide one with a better understanding for using the device in an applied application.


Lab Notes and Graphics
Figure 1.1: JFET 2N5458 Output Characteristics
Figure 2.1: JFET 2N5458 Output Characteristics
Figure 1.2: JFET 2N5458 Output Transfer Function
Figure 2.2: JFET 2N5458 Output Transfer Function
Lab Notes Page 1 of 3
Lab Notes Page 2 of 3
Lab Notes Page 3 of 3
PSpice and Computer Simulations
Pspice Simulation Graph 3.1
Pspice Simulation Graph 3.2
Pspice Simulation Graph 3.3
Pspice Simulation Graph 4.1
Pspice Simulation Graph 4.2
Pspice Simulation Graph 4.3

Electrical Engineering lab key words: JFET, junction field effect transistor, JFET Model, JFET optimizing, Q-Point, common source output characteristics, 2N5457, 2N2458, 2N5459, general purpose JFET, n-channel, transfer characteristics, drain, gate, source, transresistance, transconductance, and differential amplifiers. Offset voltage, bias current, offset current, two port models, frequency response, transfer characteristics and transfer functions, nonlinear distortion and nonlinear devices.

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