Selecting a Transistor for Sample and Hold Circuit

[sailmike]

This is my first time here on this forum. I am building an analog to digital converter. It will be running at a frequency of 12MHz and the sample and hold circuit will be 1/8th of that or 1.5MHz. This will be an 8-bit converter and Vdd will be 5 volts. I have a few parts to workout, the sample and hold circuit, comparator, and connectors to the analog input and digital output. I need to select transistors for the sample and hold circuit. I'd just like some help on how to select a transistor for my project. Below is a photo of the circuit I plan to use. The switch will be a transmission gate CMOS switch. I need to find a transistor for this switch with low capacitance in the right place. Also, need several more transistors for the amplifiers. Since, I plan to use these transistors on both a breadboard and PCB, I'll need one that's available in both through hole and SMD types. There are so many transistors to choose from that I don't know if I can find better than whatever one I find. I hope you guys can point me in the right direction to finding one via Mouser or Digikey.

I hope to use this circuit for actual projects in the future. The particular design I choose for this was for the speed, low power dissipation, wide bandwidth, high input impediance, and low output impediance.

Thank you in advance for your help,
Mike

 

[KataKoniK]

Hello Mike,

Welcome to the forum! Building an analog to digital converter is a great project and I'm happy to help you with selecting transistors for your circuit.

When selecting transistors for a sample and hold circuit, you want to look for ones with low leakage current and low parasitic capacitance. This will help minimize any errors in the sampling process.

For the switch in your circuit, I would recommend using a MOSFET transistor. MOSFETs have a low on-resistance and low capacitance, making them ideal for switching applications. You can search for MOSFETs on Mouser or Digikey using the criteria of low on-resistance and low capacitance, and also make sure to check the datasheet for the maximum voltage and current ratings to ensure it can handle your circuit's requirements.

For the amplifiers, you can also use MOSFETs, but you may also want to consider using operational amplifiers (op-amps). Op-amps have high input impedance and low output impedance, making them suitable for amplifying small signals. You can search for op-amps on Mouser or Digikey using the same criteria of low input and output impedance.

In terms of packaging, through-hole transistors are generally larger and easier to work with on a breadboard, while SMD transistors are smaller and more suitable for PCBs. You can search for both types on Mouser or Digikey and compare their specifications to see which one best fits your needs.

I would also recommend checking out online forums or communities for electronics enthusiasts to see what transistors they have used in similar projects and their experiences with them.

I hope this helps guide you in selecting the right transistors for your project. Let me know if you have any further questions. Good luck with your project!