36 votes
Accepted

Why does a single AND gate need 60 transistors?

There can be several reasons why more than the minimum 6 MOSFETs (4 for an NAND + 2 for an inverter) are used in this IC: As stated in the datasheet: The internal circuit is composed of multiple ...
Bimpelrekkie's user avatar
  • 80.9k
35 votes

Why (not) put a resistor on FET gate?

The gate source is essentially a capacitor. So with this high resistor, it would take a very long time to charge. The MOSFET will only turn on when the gate capacitor is charged above some level (the ...
Darko's user avatar
  • 899
34 votes
Accepted

Is it good practice to run large amounts of current through a MOSFET?

Why can a thick copper wire handle a large current? Because it has a low resistance. As long as you keep the resistance low (switch the MOSFET fully on, for example use Vgs = 10 V as in the datasheet ...
Bimpelrekkie's user avatar
  • 80.9k
30 votes
Accepted

Can you make a CPU out of electronic components drawn by hand on paper?

If the gain of a single inverter is less than unity, then it will not be possible to combine any significant number of gates together to build a larger circuit. The signal levels will just peter out. ...
Dave Tweed's user avatar
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27 votes
Accepted

Why have both: BJT and FET transistors on IC output?

Paragraph 2 of the description says: FAN3100 drivers incorporate MillerDriveTM architecture for the final output stage. This bipolar-MOSFET combination provides high peak current during the Miller ...
Transistor's user avatar
  • 176k
24 votes

Why (not) put a resistor on FET gate?

Big resistors on the gate slow down the switching of the MOSFET. This is OK when you are using the MOSFET as a switch (ON-OFF) but when you're driving a motor at a 20 kHz frequency and above, ...
fhlb's user avatar
  • 1,470
17 votes
Accepted

Logic-level transistor not switching

If the lamp is really 0.75A @ 5V, then it's resistance is closer to 7 ohms (when hot). The Vgs of 2.5V is the threshold, where it will just start to conduct, so you have not chosen a good MOSFET. ...
Mattman944's user avatar
14 votes

Why are the advantages of JFET over MOSFET, or why are JFET still used?

The JFET has several advantages over the MOSFET. The most important are: higher gain lower noise These are the overriding factors when building preamplifiers for low-level signals, such as those ...
Dave Tweed's user avatar
  • 173k
13 votes

Why is a high value resistor necessary for grounding a MOSFET gate?

So, why not just connect the gate to ground with a wire? How then would you switch it on and off? If the gate is permanently tied to 0V, you would never be able to tell this transistor to conduct. ...
DerStrom8's user avatar
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12 votes

Why (not) put a resistor on FET gate?

Edit: Re-interpreting the value shown on the datasheet. The resistance shown is not M\$\Omega\$, much lower, more like 3400 ohms based on the change in the switching times with external gate resistor. ...
Spehro Pefhany's user avatar
12 votes

Why is a high value resistor necessary for grounding a MOSFET gate?

Maybe you’re overthinking this. The diagram you show implies that there is a gate signal connection that is open. Rg is to ensure that the gate has a DC path to source (GND) in the absence of a gate ...
hacktastical's user avatar
  • 54.1k
12 votes

Why would you use a shared-gate multi-channel MOSFET device?

The manufacturer says it is useful in "smart phones" and as a "load switch". The lack of any matching specifications would tend to indicate it's intended for switching only. Maybe ...
Spehro Pefhany's user avatar
11 votes

Why (not) put a resistor on FET gate?

There is another reason to put a series resistor in front of a MOSFET gate - to deliberately slow down switching. This helps minimise slew rates in the circuit and hence can reduce conducted and ...
stefandz's user avatar
  • 4,152
11 votes

Using transistor as switch, why is load always on the collector

A simple reason to have the load on the collector is that it keeps the base current independent of the load. That makes it much easier to reliably keep the transistor saturated. If the load is on ...
JRE's user avatar
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11 votes

Why have both: BJT and FET transistors on IC output?

The CMOS and BJT output stages are combined to from one stage, the manufacturer calls this a "MillerDrive(tm)". Why they do this is explained in the datasheet: My guess is that they want to achieve ...
Bimpelrekkie's user avatar
  • 80.9k
11 votes

Logic-level transistor not switching

I found the FQP50N06L to have a max Vgs of 2.5V, so it should work (right?). 2.5 V is not the 'max' Vgs, it is the threshold voltage at which the FET should draw at least 250 uA when Vds is 2.5 V, ...
Bruce Abbott's user avatar
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11 votes
Accepted

Why are bipolar transistors used for switching applications?

1: Bipolars are cheaper So you can do larger experiments with the same budget. 2: Bipolars don't die an instant death from ESD So there's less risk of the user giving up on electronics because "...
Jasen  Слава Україні's user avatar
11 votes

Can you make a CPU out of electronic components drawn by hand on paper?

Paper transistors or not, a discrete transistor CPU can be made to be a reasonable size. On 15 November 2006, the 35th anniversary of the 4004, Intel celebrated by releasing the chip's schematics, ...
user4574's user avatar
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10 votes
Accepted

High voltage FET or other transistor

For highest voltage per device you'll have to look into "vacuum state transistors" (ie, tubes) like Thyratrons, or even not-so-vacuum-filled valves like the Mercury Arc Rectifier, which is ...
bobflux's user avatar
  • 77.6k
10 votes

Are there two sorts of *geometry* of JFET?

From National Semiconductor Discrete Semiconductor Products Databook 1989, an illustration of top-contacts of the N-channel FET (process 55) suggests that the gate does go underneath. This ...
glen_geek's user avatar
  • 25.9k
9 votes

Why (not) put a resistor on FET gate?

A gate series resistor can be used if a zener diode is also used to limit the gate source voltage to less than the Vgs rating of the MOSFET. The typical rating is 20V, and a 10V or 15V zener would be ...
user4574's user avatar
  • 12.2k
9 votes

Why have both: BJT and FET transistors on IC output?

Notice how the top NPN can only make the output reach VDD-0.7 V, I assume it is the job of the mosfet to take care of the last 0.7 V. It looks as if the BJT's are doing most of the grunt work and ...
Harry Svensson's user avatar
8 votes

Can I use FET input capacitance to store a charge for some time?

Why don't you just forget the fet and charge a 100 nF capacitor up? It seems that M1 is bringing nothing to the party but complication. Once charged via the diode, it will hold that charge for long ...
Andy aka's user avatar
  • 458k
8 votes

Why does a single AND gate need 60 transistors?

From ON Semiconductor MC74VHC1GT00 - Single 2-Input NAND Gate Product Guide: The internal circuit is composed of multiple stages, including a buffer output which provides high noise immunity and ...
StainlessSteelRat's user avatar
8 votes
Accepted

Are there two sorts of *geometry* of JFET?

The first sketch shows the principle of a JFET (in a way that is easy to analyze mathematically). The general concept (in any JFET) is that the depth of the conduction region (between S and D) gets ...
jp314's user avatar
  • 18.9k
8 votes

Using a Zener diode MOSFET gate voltage control circuit

it does not work. The gate voltage is 4.3V That's quite correct. Gate voltage will be about 4.3 volts in the circuit you implemented. This is because Q1 is an emitter follower and the source voltage ...
Andy aka's user avatar
  • 458k
8 votes
Accepted

Connecting FETs to make a push pull GPIO

In the case of bipolar transistors in that role: simulate this circuit – Schematic created using CircuitLab The base-emitter junctions of these transistors are just diodes. Look carefully, for ...
Simon Fitch's user avatar
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7 votes
Accepted

Insulated gate SCR, does it exist?

Apparently MOS-controlled thyristors were commercially available briefly, but were withdrawn due to performance issues. The structure looked like this:
Spehro Pefhany's user avatar
7 votes

FETs: When does Input Capacitance and Gate Charge matter?

am wondering if these two characterstics even matter if the FET is only really used in a DC, "almost always on" circuit. Probably not a big worry but, if you are interrupting a load current of ...
Andy aka's user avatar
  • 458k

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