![]() ![]() In addition, this circuit is used to transform impedances. This circuit is also commonly called a "stabilizer". The analogous bipolar junction transistor circuit is the common-collector amplifier. Because of its low dependence on the load resistor on the voltage gain, it can be used to drive low resistance loads, such as a speaker. In this circuit (NMOS) the gate terminal of the transistor serves as the signal input, the source is the output, and the drain is common to both (input and output), hence its name. In electronics, a common-drain amplifier, also known as a source follower, is one of three basic single-stage field-effect transistor (FET) amplifier topologies, typically used as a voltage buffer. ( April 2009) ( Learn how and when to remove this template message) Please help to improve this article by introducing more precise citations. It all depends on your application.This article includes a list of general references, but it lacks sufficient corresponding inline citations. This can be as simple as another transistor (usually a cheap BJT) and a resistor, or a complicated arrangement designed to switch the MOSFET at very high speeds which might include dozens of components and maybe even additional voltage supplies. Or, you can use a P-channel MOSFET, where the drain must be at a lower potential than the source.Įither way, you will likely need some sort of level-shifting circuit to translate your 0V-3V logic input to whatever levels are required by the MOSFET arrangement you select. You will have to find a way to place the MOSFET in your circuit such that the drain is at a higher potential than the source. More on the body diode: MOSFET: Why the drain and source are different? This diode is reversed biased under normal usage, but if you put the drain at a lower potential than the source on an N-channel FET, you will, as you have discovered, forward-bias this diode. However, as I explained above, it won't stay at 0V after it's operation which is why I'm trying to crowbar it to ground on demand.Īll MOSFETs inherently have a body diode between source and drain. The line I'm trying to deal with "VCOM" actually travels between -15V and +15V during it's normal operation and ends at 0V. Simulate this circuit – Schematic created using CircuitLab I've had some joy by using a single JFET but it's not as good as a MOSFET as the JFET conducts more than a MOSFET when it's "off". ![]() Obviously this shouldn't be allowed to happen however the leakage problem is currently out of my control hence why I'm trying to come up with a fix just in case I'm stuck with it.Īlso, the level of the GPIO control signal can be active-high or active-low. To further explain my problem: I need to discharge a capacitor at a specific time (when an IC turns off) clamping it to ground, otherwise other voltage lines held up by capacitors begin to leak into it causing havoc (specifically the negative voltage line). Is there any transistor suitable for this application or must Vg always lie between Vd and Vs? Also, I'm also trying to keep the circuitry as minimal as possible. This works as expected with a drain of +10V but with a drain of -10V the FET just acts like a diode, dropping only 0.6V across itself or nothing. With the positive-going Gate voltage, I tried an NMOS FET. I'm trying to create a switch between a -15V and +15V supply using a gate signal of 0V to +3V. ![]()
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