dual-gate mosfets - logic AND? (BF998) - Page 1
Forum section: electronics / beginners

author: pstrag
author_rank: Newbie
timestamp: October 07, 2013, 04:00:45 am
content: I'm trying to understand how dual gate mosfets work.Can I use them as logic AND gate?If so, what is the input capacitance when I will connect one gate (G1) to the Drain and I will use only second gate G2 in typical MOSFET configuration? In datasheet of BF998 I found it will be approx. 2pF so it looks like ideal low-power MOSFET, or am I wrong?Thanks, Slawek, Poland.

author: Bertho
author_rank: Regular Contributor
timestamp: October 07, 2013, 09:53:37 am
content: Quote from: pstrag on October 07, 2013, 04:00:45 amThey are more like a mixer.The best way to see what they do is to look at the graphs "Output characteristics" and "Transfer characteristics" in the datasheet (figs 5 and 6 of the NXP datasheet).One thing you should notice is that if both Vg1 and Vg2 are 0V, then there still is an Id current (transfer graph). These transistors are made in such a way that you can easily mix two analog signals (f.x. VCA).These mosfets would suck at digital logic. You'd need negative voltages to quench the output.

author: Avi
author_rank: Contributor
timestamp: October 10, 2013, 08:30:30 am
content: The October edition (current) of the silicon chip magazine explains in the sidradio article, the 2 gates of the BF998 are used such that the DC level on 1 gate can effect the amplification of the signal on the other gate, and supporting negative voltage makes it optimal for signal input. However i'm not sure if this also causes a dc offset of the output which must by removed with a bypass cap. Perhaps someone has details on the specifics?

author: zapta
author_rank: Super Contributor
timestamp: October 10, 2013, 04:25:44 pm
content: Quote from: Bertho on October 07, 2013, 09:53:37 amWhat does a mixer do? Adding two signals? Multiplying two signals?

author: SeanB
author_rank: Super Contributor
timestamp: October 10, 2013, 07:02:10 pm
content: I did use a 3N201 as a gate to extract data from a VFD display ( VCR display) to do controlled switching. As the display used -25V to drive the display and the voltages swung from -25V ( segment or anode held off) to +5V ( display segment or anode on) and this was done by the main microcontroller chip internally this was the only way to get the data. I used a resistive divider of 2 470k series resistors to keep the voltage below the gate max voltage levels, and a simple RC filter to integrate the resulting pulses to drive the relay I was driving with this. Worked very well, and the only problems were a slight ghosting on the segment I used from the current through the resistors, only slightly visible in a dark room with the display off.

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: October 10, 2013, 08:00:19 pm
content: Quote from: zapta on October 10, 2013, 04:25:44 pmA mixer presents the sum and difference of the two input frequencies at is output.One of those frequencies is normally undesired and needs to be filtered out with a tuned circuit or filter.

author: Bertho
author_rank: Regular Contributor
timestamp: October 10, 2013, 08:06:06 pm
content: Quote from: zapta on October 10, 2013, 04:25:44 pmEhm, a mixer mixes signals...VCA: Voltage Controlled Amplifier. Which effectively means it multiplies. You can trace the behaviour from the diagrams in the datasheet. However, the exact schematic use can do many funny things. The details are always in the setup where it is used, but the basics are multiplicative.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 11, 2013, 02:41:51 pm
content: Another thing a dual gate MOSFET is useful for is AGC controlled amplifiers.Application of AGC to single gate FETs & BJTs is a bit messy,& entails extra components compared to the dual gate device.