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*H Bridge
 
*H Bridge
  
An H bridge is an electronic circuit that causes current to flow in one direction or the other ( from a single ended power supply ).  Often used for motor control [[motor driver]].
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An H bridge is an electronic circuit that causes current to flow in one direction or the other ( from a singel ended power supply ).  Often used for motor control [[motor driver]].
 
It is an electronic double pole double throw switch.
 
It is an electronic double pole double throw switch.
 
**[http://code.rancidbacon.com/Electronics] See Section on ''H-Bridge''
 
**[http://code.rancidbacon.com/Electronics] See Section on ''H-Bridge''
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This high side switch usually requires the base voltage of Q to be VPLUS_VDD plus the turn-on voltage of the transistor to turn all the way on. Another approach to the high side switch that requires a lower turn-on voltage is to use a PNP transistor as the switch. The base of the PNP is pulled up to VPLUS_VDD and connected to the collector of a small signal NPN transistor, Q2. Q2's emitter is connected to ground and its base is connected to the input signal through a current limiting resistor -- now the problem is that a high voltage is required to turn the switch off.
 
This high side switch usually requires the base voltage of Q to be VPLUS_VDD plus the turn-on voltage of the transistor to turn all the way on. Another approach to the high side switch that requires a lower turn-on voltage is to use a PNP transistor as the switch. The base of the PNP is pulled up to VPLUS_VDD and connected to the collector of a small signal NPN transistor, Q2. Q2's emitter is connected to ground and its base is connected to the input signal through a current limiting resistor -- now the problem is that a high voltage is required to turn the switch off.
 
 
=== bootstrap circuit ===
 
 
Often H-bridges use n-FETs in all 4 arms, to reduce cost.
 
Unfortunately, power n-FETs require a gate voltage much higher -- many power n-FETs require 10 V higher -- than both of their other two legs in order to keep them turned hard on (necessary for efficient power H-bridges).
 
Since the drain of the high-side n-FET is generally already connected to the highest voltage available from the batteries,
 
where are we going to find that even higher voltage?
 
Often we use a bootstrap circuit.
 
(see
 
Mamadou Diallo from Texas Instruments.
 
[http://www.ti.com/lit/an/slua887/slua887.pdf "Bootstrap Circuitry Selection for Half-Bridge Configurations".
 
2018.
 
)
 
 
Historically
 
The Intel 4004 uses a bootstrap circuit[http://insanity4004.blogspot.com/2015/10/puzzling-out-bootstrap-load_13.html]
 
which is apparently one of several reasons
 
the 4004 has a minimum clock rate (maximum cycle time).
 
Reece Pollack is translating the design to a static-logic implementation[http://insanity4004.blogspot.com/2012/09/full-circle.html].
 
(a fully-static system makes it possible to pause the system indefinitely,
 
which is very convenient for debugging).
 
Today we have several alternatives to bootstrap circuits:
 
* If you're building digital logic out of discrete transistors, you might as well replace the 2-transistor bootstrap load circuit with an actual physical discrete resistor, which works better (?).
 
* If you're building digital logic out of FPGAs or full-custom ASICs, you're probably using CMOS -- both nFET and pFET -- and a single nFET works better than a 2-p-FET bootstrap load, and a single pFET works better than a 2-n-FET bootstrap load.
 
* If you're building a H-bridge, even today it is often better to use all-n-FET rather than both n-FET and p-FET; the bootstrap circuit gives a minimum PWM frequency (maximum PWM cycle time) and a minimum and maximum duty cycle (one step less than 100%); replacing that circuit with an independent oscillator and charge pump allows you to go all the way to 100% forward or 100% reverse.
 
 
 
  
 
== Transistor Emitter Follower ==
 
== Transistor Emitter Follower ==

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