# Opposite of a transistor

So a NPN transistor has 3 legs. If you connect up leg 2 and take current away from it then current can cross over from leg 1 to 3. What I want is when you take away/give current to the central leg current cannot pass from 1 to 2 but can when no current is given/taken from the centre. Look at the schematic below to see what I would like to know exists.

simulate this circuit – Schematic created using CircuitLab

• "piece"? "middle current"? I don't understand what these terms mean in the context of transistors – Andy aka Mar 29 '13 at 10:11
• Identify "legs"! Correct terminology is emitter, base and collector. – Leon Heller Mar 29 '13 at 10:26
• If you use two transistors (in a certain type of circuit) you can apply current to leg 2 of one transistor and it stops the current between legs 1 and 3 of the other. Does this achieve what you want? If not then you need to rephrase your question or rethink what you are asking. – Andy aka Mar 29 '13 at 11:21
• Or maybe you are searching for a JFET? When you bias the gate (not always the middle pin or pin2) it will restrict the ability to flow current between the other two pins – Andy aka Mar 29 '13 at 11:25
• Identifying transistor pins as "legs" is pretty useless to indicate their function. You numbered the "legs" 1-3, but even if that were meaningful, you haven't told us what package and which one is pin 1. Then there are various pinouts even between transistors in the same package, so even knowing which pins you means sticking out of the package doesn't tell us anything. We use the terms "emitter", "base", and "collector" to describe the functions of the three pins of a bipolar transistor. – Olin Lathrop Mar 29 '13 at 12:36

A J110 N channel JFET will do what you ask. Source, drain and gate are connected as shown: -

Source is ground, drain is a load resistor connected up to (say) 5V. Gate is the control pin and should be taken negative to source(0V) to turn off the transistor.

The J110 JFET has a Vgs(cut-off) of somewhere between -0.5V and -4.0V. This means that if gate is at 0V the device has good conductivity between drain and source and will pass current through the load resistor.

BUT if the gate is taken below the cut-off voltage the load current will fall to below 10nA (according to the Fairchild spec). J110 is just one JFET. It is closely related to J108 and J109 - these have different cut-off voltages to suit different applications.

I believe that what you're looking for may simply be a PNP transistor. Your specification is not very clear and you're conflating current and voltage, which isn't good.

An NPN transistor will conduct collector-emitter current as we raise the voltage on the base.

A PNP transistor (placed "upside down", so to speak, into the circuit, of course) will conduct collector-emitter current increasingly as we lower the voltage on the base.

In the schematic below, as we raise the voltage on input IN1 above around 0.6V, the NPN transistor starts to conduct. It cuts off below that voltage. But IN2 behaves differently. The PNP starts to conduct as we lower the voltage below $V_{cc} - 0.6$. It is cut off at higher voltages than that.

When a PNP is used in a positive-suply voltage circuit like this, a high input cuts off current, and a low input turns it on, opposite to the behavior of an NPN.

So, the current follows voltage in the NPN circuit, but is inverted in the PNP. That's current. Whether or not the output voltage is inverted depends on where you take it. If you take as your output the voltage on either collector resistor, then both transistors behave as inverting switches/amplifiers! That's because although the current control behavior is opposite between the two, the collector resistor also changes places between the two: it is on the top in the NPN circuit, but bottom in the PNP circuit. So it develops a $V_{cc}$ referenced voltage in the NPN on, but a ground-referenced voltage in the PNP one.

If you want the output voltage to swing in the same direction the input, you take it from the bottom resistor (emitter resistor on the PNP, or collector on the NPN). If you want the output voltage to swing in the opposite direction, take it from the top resistor (collector resistor on NPN, emitter on PNP).

In other words, a common-collector circuit is always inverting, and an emitter-follower always noninverting, regardless of whether we build them using NPN or PNP, or whether we use a negative or positive voltage supply.

In addition, there are other kinds of transistors like Junction Field-Effect Transistors (JFETS) and MOSFET, which come in N-channel or P-channel varieties and can be "enhancement mode" or "depletion mode". If you know about all the transistor types, you can make a better informed choice about what will work well in your circuit.