Calculating Transconductance for a BS170 MOSFET

The BS170 MOSFET is one of the most common transistors used today by the electronic designer and hobbyist. Along with the lower-powered 2N7000, Its most common uses are as a switch to turn on various devices such as small motors when they are connected to microprocessors such as the Arduino. Small-signal MOSFETs such as the BS170 can also be used as voltage amplifiers in the inputs of preamplifiers. Lower noise devices have been developed that are more suitable for professional applications. One of the features that make this device so popular is the ease of use. Unlike its cousin the bipolar transistor, the BS170 needs almost no power to turn it on and an effective switch or amplifier can be designed with fewer components. The MOSFET combines the best features of the vacuum tube and the bipolar transistor into one package; that being the high input impedance of the vacuum tube and the low output impedance of the bipolar transistor.

Step 1: Technical Level

This experiment requires an intermediate understanding of electronics and would is aimed at the advanced high school electronics student or first-year college or university. The student needs to have good soldering skills, know how to use a multimeter to take measurements of voltage and current and how to handle static-sensitive devices such as a MOSFET safely.

Step 2: Objective

The objective of this lesson is to connect a BS170 MOSFET to a 10K potentiometer with the center wiper attached to the gate and the top terminal connected to +9 volts and the bottom terminal connected to negative or ground. The source terminal will be connected to ground and the drain terminal will be connected to +9 volts through a multimeter connected in current mode. Another multimeter will be used in voltage mode to monitor the voltage presented to the gate as the potentiometer is used to change the voltage to vary the current going through the device. The input voltage will be recorded and plotted against the output current. The change in output current will be divided by the change in input voltage and the device DC transconductance will be calculated. This will then be compared to the manufacturer's published transconductance.

Step 3: Parts and Test Equipment Needed

1) BS170 MOSFETs. Electronics store or online

2) 10K potentiometer. Electronics store or online.

3) 2 Multimeters. Electronics store or online.

4) 9 Volt Battery. Hardware, entertainment or electronics store.

5) Ten small Alligator clips with connecting wires. Electronics supply store or online.

6) Perfboard. Electronics store or online.

10) Solder. Electronics store or online.

11) Two multimeters. Electronics store or online.

Step 4: Understanding MOSFET Terminology

The BS170 MOSFET is a voltage-controlled device with three terminals, gate, drain and source. The gate sees only voltage and no current is used by it. This voltage is known as Vgs or the gate-source voltage. This makes it a "high impedance device". This can easily be 10 megohms or more! A current that flows through the device from source to drain and the amount that flows through it is controlled by the voltage appearing between the gate and source. This current is called Id or drain current. The output of the MOSFET is a low impedance, which can be as low as 1 ohm. The BS170 is an enhancement mode device, meaning it needs to be turned on or "enhanced" for current to flow through from the source to the drain. Depletion mode devices work the opposite way. As can be seen on the graph, the Vgs voltage is gradually increased until Vth is reached, which is the threshold voltage needed to turn the device on. As long as Vgs is greater than Vth, we are in the saturation region, below that it's called the cutoff region. If the device is to be used as a switch, a certain amount of current will be needed to turn on a motor or other device on and an appropriate Vgs or input voltage can be calculated for that given output current. The BS170 can also be used as an AC voltage amplifier. To give a low distortion output, the Vgs voltage would be made half of the Vds voltage. The Vds voltage is the total applied voltage across the device from source to drain. Vdd is the equivalent of Vcc in a bipolar transistor circuit or the total applied voltage to the circuit. A link to the datasheet can be found here:

https://www.onsemi.com/pub/Collateral/BS170-D.PDF

Step 5: Set Up Equipment

The potentiometer and the MOSFET can be connected on the perf board as shown in the picture and schematic and soldered underneath so they can make a good connection. The multimeters can be connected as shown in the next picture with the alligator clips and once the connections have been checked, everything can be connected to the 9 -volt battery. Make sure one meter is connected in the voltage mode and the other is connected in the current mode.

NOTE: The BS170 device is static sensitive and can be destroyed by simply touching the gate terminal with your hand. Use a wrist strap when handling the device and once it's ready to be soldered, use a grounded soldering iron to solder the device to the potentiometer. Also, take care to use a heatsink when soldering to divert heat away from the leads of the device. Long-nosed pliers are excellent for this purpose.

Step 6: Record Data and Graph

Gate Voltage------------- Drain Source Current in milliamps
0------------------------------------------- 0

.5------------------------------------------- 0

1-------------------------------------------- 0

1.5-------------------------------------------0

2.0 ------------------------------------------4

2.5------------------------------------------ 75

3.0------------------------------------------167

3.5------------------------------------------290

4--------------------------------------------425

4.4------------------------------------------490

Step 7: Calculate Transconductance and Compare to Specification Sheet

From the data on the graph, we will take a part of the slope which has linear characteristic and insert the figures into the above formula to calculate the transconductance:

gm in siemens = change in drain current divided by change in gate-to-source voltage

(.290 - .75 amps) divided by (3.5 - 2.5 volts)

.215 divided by 1

= .215 Siemens or 215 millisiemens

This compares favorably with the transconductance given on the manufacturers data-sheet for which the BS170 is rated at 200 millisiemens.

Note: The older measurement for conductance mho or mmoh has been replaced by the siemen or millisiemen. They mean the same thing.

Step 8: Conclusion

In this instructable, we have been shown how to create a testing circuit for the BS170 MOSFET to allow us to record input voltage Vgs against Id or drain current. We took down the readings and plotted them against each other on a standard x/y graph. A linear region was picked on the graph and a change of current was divided against a change of voltage to calculate gm or device transconductance. It was also explained that the device could be used as a switch or a voltage amplifier.