Transmission Gate in LTSPICE

Question

I am trying to simulate a transmisson gate in LTspice but regardless of the state of the clock it keeps giving me the exact copy of input as an output. I tried inverting the clock with a inverter but it didn't also change the output. It might have something to do with the threshold voltages of Mosfets but I couldn't find where I can control them since the only parameters of Mosfets were Drain-Source Voltage,On-Resistance and Gate Charge.

Edit: I also tried it with a load before but it didn't work as well.

Answer 1

This is actually a common problem due to electronics school programs not making it clear that MOSFETs are actually 4-terminal devices. In integrated circuit textbooks, they'll usually explain in the intro chapter that the body terminals for N-channel MOSFETs are always assumed to be connected to VSS with the same for P-channel MOSFETs and VDD. This allows them to be able to draw non-cluttered schematics for the rest of the book by only drawing out 3-terminals for each 4-terminal MOSFET. Only in special cases where the VSS/VDD rule isn't true they'll draw a 4th terminal on that MOSFET and connect it somewhere else.

Non-integrated circuit (discrete) MOSFETs which you can grab from a parts bin are a special case of a 4-terminal MOSFET where the source and body terminals are effectively tied together. This is a simplified way of looking at it as the actual MOSFET construction between the two is different, but it suffices for now. The confusion arises when a student tries to make a discrete version of a circuit from an integrated circuit textbook, such as a transmission gate. The textbook has all its schematics drawn with 3-terminal MOSFETs, so it seems logical to just build it in discrete form with off the shelf 3-terminal MOSFETs. But you can't do this, as you need the body terminals connected to VSS/VDD but they're permanently tied to the sources instead in these MOSFETs.

Although you can't build the circuit out of typical discrete MOSFETs, you can still simulate it. The ICE in SPICE stands for "integrated circuit emphasis" so SPICE perfectly capable of doing this. You just need to add 4-terminal MOSFETs and connect the body terminals to the proper locations. In LTspice, you would use the nmos4 and pmos4 symbols. You also need to provide a .model statement for each MOSFET type so you can set the threshold voltage, Vto, and effective ON resistance, Kp, (at the minimum). Below is my duplication of your circuit with the required changes.

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A couple extra things to note. First, although you can't build transmission gates from typical discrete MOSFETs, you can still build different kinds of analog switches. I didn't mention it above, but there's a side-effect where once the body and source terminals are tied together, there is a parasitic diode connecting source and drain (called the "body diode") you must take into account. You can put two N-channel or two P-channel MOSFETs in series with these diodes counter to each other to mitigate this. However, with a switch like this the control voltage is referenced to the MOSFET sources which will change with the content of the signal you're trying to pass/block so it's quite tricky to use this method and only works in certain conditions. Discrete MOSFETs also have high terminal capacitances so this method only works with audio frequencies and lower otherwise the signal can shoot through the MOSFETs while they're OFF. JFETs are usually better in this aspect, but they suffer from a much higher ON resistance so that's their tradeoff.

Second, there is a company called Advanced Linear Devices which provide integrated circuit MOSFETs for generic use with all 4 terminals provided. You can use these to make transmission gates. Lastly, the CD4007 integrated circuit has MOSFETs where their body terminals are exposed to the pins on the chip. You can use these to make your own transmission gates as well.