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JFET Amplifer and Filter Project

Posted 17th May 2017 at 10:37 PM by SyncTronX
Updated 17th May 2017 at 11:36 PM by SyncTronX (add images)

JFET Amplifier and Filter Project.


A lab was developed with something a little bit more hands on for participants than just blindly doing the NIDA labs. We would benefit from making an N-Channel JFET amplifier and asked if we would be interested. Of course, we weren’t told the JFET type or gain and didn’t know we’d also include a bandpass filter. These parameters we’d have a drawing from which to choose. Finally, we’d have to document and report the entire process in APA format.

Each project is unique and provided on two stapled pages placed upside down in a mixed pile at the front of the room. Each participant was asked to come up and draw their project. I drew project labeled “E” on the upper right corner of the second page. Included were the following: JFET 2N5485 or 2N5458, Gain set to 22, the Bandpass to be 15kHz – 30kHz, and Total Circuit DC Power as Calculated/Measured.
Other resources were available as we determine their need, they are included in the Reference section. I will determine which JFET to use after analyzing the data sheets. After selecting the JFET, I’ll include specifications, bias and operational point(s) then calculate the specifications for the Bandpass filter. This will all be planned and documented according to the following deadlines:
Phase 1 – Week 1
Phase II – Week 2
Phase III – Week 3
Project Design

Each phase has its own deliverables which follow:
Phase I – Week 1

Read the multi-week lab objectives. Identify the N-Channel JFET with data sheet. Calculate the following performance expectations including the following:
· DC operational parameters
· AC operational parameters
· Detailed schematic
· Transconductance graph
Performance Expectations

I drew letter “E” from the pile, it contained the following:
· JFET: 2N5485 or 2N5458
· Gain: 22
· Bandpass: 15kHz – 30kHz
· Total Circuit DC Power: As Calculate/Measured

The two JFET choices, 2N5485 or 2N5458, have data sheets which are included in the References section. The 2N5485 is an N-Channel Radio Frequency (RF) Amplifier while the 2N5458 is an N-Channel General Purpose (GP) Amplifier. The 2N5485 was listed first in the drawing, perhaps the reason is availability.
Data Sheet Review

2N5485 – N-Channel RF Amplifer (RF) is listed as designed for electronic switching applications such as low on resistance analog switching.
2N5458 – N-Channel General Purpose Amplifier (designated as GP) is listed as a low level audio amplifier and switching transistor and can be used for analog switching applications.
The Absolute Maximum Ratings are identical for both JFETs, this is where the similarities end. The GP provides for higher device dissipations 625mW vs. 350mW for the RF device. The Off Characteristics are similar, On Characteristics, Small Signal Characteristics however, we still need to consider how the device will be used and the data sheet that speaks the loudest to me as the proper choice is the 2N5458 – N-Channel General Purpose Amplifier. This is what the experiment is about. 2N5485 – N-Channel RF Amplifier is selected because it’s available, the 2N5458 GP is not.
DC Operational Parameters

The DC operational parameters were calculated after the JFET was selected, in this case the 2N5485. The data sheet was analyzed and calculations were made based on the work we covered during the course. This included calculating the parameters then drawing them in a Transconductance graph. Then this lead to the schematic drawing the base circuit. Here the first bias level calculations are shown in Figure 1, (attachments, page – 7 –).
The data sheets showed different maximum drain currents where ID = IDSS, where Vgs = 0. They were 10mA and 6mA, for the Transconductance Graph you can see that 6mA was chosen. This ended up being an error and 7mA should have been used instead. But it didn’t matter, because for the 2N5485 JFET that was selected the calculations didn’t point to the correct operational point. I think the actual JFET must lie outside the datasheet parameters.

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figure 1.

From the calculations (Figure 1) the basic circuit was drawn as shown in the upper left side of the graphic. Transconductance graph was also an input to the DC operating parameters. It should also be noted the entire process is iterative, not top down, meaning that you have to go back and revisit your calculations, work through them, and tweak the circuit on which we are working.
In this case there were four different prototypes developed. The first prototype was so far off I concluded that I needed make a variable drain and source resistor circuit to identify proper circuit values. This variable prototype led me to develop the final working JFET amplifier, the one with the “correct” parameters.
AC Operational Parameters

The AC operational parameters are composed of two parts. The first part is the circuit gain of the JFET amplifier while the second part is the Bandpass filter. The Bandpass filter is composed to two parts, a high-pass filter and a low-pass filter, wired in series. Each part was calculated then tested. There was a substantial amount of work on the circuit which far exceeds the calculations.
The first amplifier circuit test using the calculations for AC gain, AV = VOUT ÷ VIN showed a gain of 10. Less than half of what it should be. I chose a VIN that would be easy to follow along with, 100mV. For gain to be 22, with VIN of 100mV, I should look for 2.2V at the output. Thus,
AV = VOUT ÷ VIN, 2.2V ÷ 100mV = 22.
The second set of AC parameters was derived by calculating and evaluating the 2nd Order bandpass filter and found in the Electronics Tutorials, Reference. Figure 2 shows major details such as frequencies, stopband, passband, and stopband. The output in dB shown vertically and frequency shown horizontally in a nice color graphic, overleaf.
Frequency Response Curve

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Click the image to open in full size.Figure 2.

The AC frequencies were part of the project drawing and for this circuit it was 15kHz for the high-pass filter and 30khz for the low-pass filter. The calculations showed which values to use for each filter. The high pass filter was calculate, then the low pass filter. I am unsure if the information provided about the band pass filter was good. The filter handouts further describing filter calculations follow and provide the detail needed to calculate the components needed for each portion of the bandpass filter. These are shown in Figure 3.

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Figure 3.

It is important to remember that the high pass filter is encountered first, as it passes everything above it, in this case it is 15kHz. Then we proceed to the right side of the graphic above and we encounter the low-pass filter, which rolls off everything below it, in this case it it 30kHz. For the calculations I started with capacitor values C1 = 0.01uf and C2 = 0.001uf (Grob’s, p788).
High-pass filter calculation, 15 kHz:

1 ÷ 2Π R1C1 = R1 = 1 ÷ (2Π * 15x10-6 * .01 x 10-6) = 530 ohms
Low-pass filter calculation, 30 kHz:

1 ÷ 2Π R2C2 = R2 = 1 ÷ (2Π * 30x10-6 * .001 x 10-6) = 5308 ohms
From the preceding calculations it is straightforward to selecting the values for the high-pass and the low-pass components. Their location in the bandpass filter is shown in Figure 4, see if you can find them in Figure 5.

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Figure 4.

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Figure 5.

There were several iterations of measuring the bandpass filter. The filter works fine when using either the high pass or the low pass. A problem arises when the filter is set in series, it doesn’t work as advertised, the compilation data is shown in the following table (Figure 6).

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Figure 6.

Detailed Schematic

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Figure 7.

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Figure 8.

There are two schematics that form the working model, these include the design of the 2N5485 amplifier (gain 22) and the bandpass filter 15kHz – 30kHz).

Transconductance Graph

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Figure 9.

The Transconductance Graph (Figure 9) was calculated based on data sheet values and textbook (Floyd) information. My choice for setting the parameters followed the textbook suggestion for a starting point just lower than center curve. A center Vgs of –2V providing headroom for drain current and Idg starting at 1.5mA. Phase II – Week 2

Move from design/calculation to Multisim computer simulation. Document performance in Multisim versus calculations. Maturing the design for performance projections.
Phase III – Week 3

Hardware build and test. Notes include using NIDA or other applicable board. Adjust components as required.
Calculation vs. Actual

The process started by calculating the values necessary to start a design based on the data sheet values and the assigned gain of 22. This is where the fun started. Floyd demonstrates several methods of using data sheet values including developing the JFET Universal Transfer Characteristic Curve (Floyd, p392.). From this curve the forward transconductance (gm) is derived, where gm equals the change in drain current (ΔID) divided by the change in voltage of the gate & source (ΔVGS). From the drain and source currents drain and source resistor values are derived.
This information was turned in week 3 as scheduled, reviewed and returned. I started designing in Multisim then realized it’s a very short design cycle to get things working. Based on experience actual designs can be tricky to work and simulations don’t always work as calculated. Better to build the amplifier and filter. Follow along as I explain in “Research is A Messy Business”.

Research is A Messy Business

Research and design is not a nice neat compilation of papers that one cranks out with little thought. Research and design is a messy business. Why? Because what we do is to figure it all out. That means much thought has to go into the process. Paper is just the starting point, which is included on the graph paper notes that I took and those reviewed.

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Exhibit 1.

Exhibit 1, below shows just what we must do to try and get things working properly. Guess what? There is also a lot of equipment involved too. There doesn’t have to be, but it can sure help diagnosing and troubleshooting.

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Exhibit 1.1
Exhibit 1.1: Tale of Two Amp Circuits Left Half, Right Half.

Exhibit 1.1 show’s the tale of two amp circuits, they are in the process of being dialed in. For some reason the circuit on the left would only perform up to the gain equal to 10. The requirement was to have the gain of 22. The pots were used in the left circuit to adjust the drain and source resistances around the center of calculated values for the proper design and specified characteristics.
But that didn’t work out too well for me and regardless of tweaking the pots nothing seemed to be working out as calculated. So I stopped the process then recreated a new circuit that you see on the right half of the breadboard (Exhibit 1.1).
The right half circuit was recalculated and in that circuit I used some slightly different components and changed the values to the new improved calculations. There was more gain, measured to be 16, still short of the required 22. What to do?
Review the source documentation and figure out a way to do this. I eventually came upon the method I was attempting for my filter, that is using sweeps to dial in the correct component values. I was also concerned about the smaller trim pots. I’m sure they could withstand the dissipation etc but who knows? I needed to sweep the values of the drain and source resistors again and create a circuit that will meet specifications. One thing that I noticed was the text described how Vgs was the control function for the JFET operating points. That could be problematic because that would involve another power supply. Now I understand why Keysight/Agilent/HP made first the dual-channel, and the tri-channel power supplies. It you work with these devices you need different levels of power to control different parts of the circuit. Three channel power supplies are going to be on my watch list in the near future.
Yes, that is just what I need. Another piece of gear. However, If I’m smart about it, I could sell off some of the gear I’m not using to pay for a good triple power supply or two. Exhibit 4 shows a resistor decade box used to dial in the drain resistor value. The two chicken head knobs sit on two potentiometers wired as rheostats in series to dial in the source resistor value.

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Exhibit 4: Method of Setting Amp Gain via Drain and Source resistor sweep.
RD = 23k Ω, RS = 5K7 Ω.

After a few different sweeps I still didn’t make the gain of 22 that was required for this circuit. Notice when you compare the exhibit photos, I went through many more changes. Look at the capacitors that were listed in some of the diagrams in the book (Floyd) covering the bias points for JFETS used for the self-biasing method.
I continued to sweep the drain and source resistors. They just peaked out. Finally after reviewing the data sheets again I saw that the absolute maximum drain to gate voltage was 25 volts. Then, the gate to source voltage was -25 volts and I knew that I could start to raise my voltages, then finding and exceeding the gain equal to 22, I continued sweeping until I settled at the maximum gain for the lowest voltage.
That voltage was identified as + 17 VDD. The sweeps dialed in the following values for the drain resistor at 23k Ohms and the source resistor at 5k7 Ohms. The gain, Av = Vout/Vin came to the following, 22 = 2.2VAC/.100VAC. This met the specifications.

Final Report and Conclusion

The conclusion moving from classroom, to lab, receiving basic design and then calculating them based on a series of formulas is only a starting point. That is a lot more that has to occur to have a working prototype.
The process is iterative, meaning that we have to go back and review our design, our notes and look up resources that we are going to use and then use them. We also cannot just use the formulas blindly thinking that is all we have to do to get things working. It won’t, it can’t.
We used simulation as part of the process also, but that has limitations too. When I realized that, and confirmed design, but got stuck troubling shooting the simulator. I made the choice that I would rather trouble shoot the circuit and not the software simulator because my past experience showed that I would still need to spend time trouble shooting the actual prototype, lay out and function.
The JFET amplifier is working. The bandpass filter is another challenger. Each part of the filter works independently from the other. The high-pass filter work, the low-pass filter works, but the band pass filter wired in series doesn’t work as advertised.
Williams discusses the issue in his work the “Analog Filter and Design Handbook” which is beyond the scope of this project.


Electronics Tutorials. (2014). Passive Band Pass Filter. Reviewed on April 29, 2017, from: https://www.electronics-tutorials.ws/.../filter_4.html.
Fairchild Semiconductor. (1997). 2N5484, 2N5485, 2n5486, MMBF5484, MMBF5485, MMBF5486, N-Channel RF Amplifier. Datasheet, originally published by Fairchild Semiconductor International, San Jose, California, U.S.A.
Fairchild Semiconductor. (1997). 2N5457, 2N5458, 2n5459, MMBF5457, MMBF5458, MMBF5459, N-Channel General Purpose Amplifier. Datasheet originally published by Fairchild Semiconductor Corporation, San Jose, California, U.S.A.
Floyd, T. (2012). Electronic Devices: Electron flow version, Ninth Edition. Upper Saddle River, New Jersey, U.S.A: Prentice Hall.
Learning About Electronics. (2017). Bandpass Filter Calculator. Reviewed on April, 29, 2017, from the following: Bandpass Filter Calculator.
Trade Paper. (2017, Spring).Including:
Characteristics of JFETS.
Configuring a Parameter Sweep Analysis in Multisim.
Multi-Week and Final Lab.
Bandpass Filter Equations.
FET Amplifiers.
JFET Frequency Response: Experiment #: 2.
Selecting JFETS, including related notes and review.
Schultz, M. (2011). Grob’s Basic Electronics, (11th ed.). New York, New York; McGraw-Hill, Companies, Inc.
Williams, A. (2014). Analog Filter and Circuit Design Handbook. First Edition. Parts published previously in Electronic Filter Design Handbook, Forth Edition. (2006). McGraw Hill, U.S.A.
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  1. Old Comment
    Nice job writing this up :0
    Posted 20th May 2017 at 07:05 AM by kasey197 kasey197 is offline
  2. Old Comment
    SyncTronX's Avatar
    Thanks Kasey.
    Posted 22nd May 2017 at 03:13 AM by SyncTronX SyncTronX is offline

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