BJT is faster than MOS-Every reply will b clicked helped

nan_ishan · Dec 13, 2007

HI guys this one is for those who think that MOS is faster than BJT.
Actually BJT is faster than a cmos.
The reason behind this is that MOS has a high input capacitance which lowers the speed .
Although,Cmos is mostly used ,it is due its other advantages....which you all know and there's no need to mention..........
Here is the proof:

https://www.allaboutcircuits.com/vol_4/chpt_3/7.html

Bipolar transistors are used preferably when large load and speed are needed:
given in:

**broken link removed**

I'll click helped to everyone who replies,just say whether u agree or not.I have clicked helped to everyone who has posted a reply(the intention is just to spread knowledge.....)

fala · Dec 13, 2007

Hello, there are other factors that are also important in determining bandwidth of a transistor. first of all it is true that input capacitance of MOSFET is higher but question is how much> with typical input capacitance of 120pF and output impedance of driver 100 Ohm it still has about 13MHz bandwidth. Some BJT transistors have even poorer bandwidth. One important factor in BJT bandwidth is speed of base-collector diode. while usually base-emitter is much faster that base collector diode, the base collector diode can restrict bandwidth of BJT. you should look at datasheet of each transistor some BJTs are faster than MOSFET some do not.
regards,

A.Anand Srinivasan · Dec 13, 2007

bjt is faster than mos... only by scaling mos has been made to approach higher speeds...

is_razi · Dec 13, 2007

mosfet has a faster turnoff time , and the gate drive circuit can be designed to supply some few extra current to rapidly turn on the mos, also note to this expression from abraham pressman in the "switching power supply design" book ,chapter 9-introduction :
"the faster switching speed of MOSFET's has permitted increasing power supply switching frequencies from about 50 to 200 or 400 KHz."
.

nan_ishan · Dec 13, 2007

is_razi said:
mosfet has a faster turnoff time , and the gate drive circuit can be designed to supply some few extra current to rapidly turn on the mos, also note to this expression from abraham pressman in the "switching power supply design" book ,chapter 9-introduction :
"the faster switching speed of MOSFET's has permitted increasing power supply switching frequencies from about 50 to 200 or 400 KHz."
.

It does'nt mention that Mosfet is faster than BJT anywhere

faraz7777 · Dec 13, 2007

As a matter of fact, the first advantage of CMOS ckt is its lower energy consumption, and the ability of usage with several levels of voltages... eg. we need 5 volt for BJT 74xxx ICs, but not necessarily for 74cxxx ones.
However, in some cases we may see a very fast modern CMOS IC which has reached good comparable speeds with JBTs.

INS-ANI · Dec 13, 2007

Nice find.
Thanks

nikhilele · Dec 13, 2007

not only MOS takes less space, less power, it can work on lower voltages...

ehsanelahimirza · Dec 13, 2007

MOS is faster then BJTs as i think it for the switching delays.
for BJTs certain amount of current is to b maintained if we have to driver some circuit and that will depend upon the B(beta of BJT)

but for CMOS we can get sharp rise and falls of voltage for its switching as compared to BJT which will take some delay to aquire a value of current

nan_ishan · Dec 14, 2007

ehsanelahimirza said:
MOS is faster then BJTs as i think it for the switching delays.
for BJTs certain amount of current is to b maintained if we have to driver some circuit and that will depend upon the B(beta of BJT)

but for CMOS we can get sharp rise and falls of voltage for its switching as compared to BJT which will take some delay to aquire a value of current

did u check out d links posted by me

SWINI · Dec 14, 2007

did u check out d links posted by me
can u post them again to the below given addr please
"parimisys@gmail.com"

nan_ishan · Dec 14, 2007

SWINI said:
did u check out d links posted by me
can u post them again to the below given addr please
"parimisys(at)gmail.com"

I have sent the links to ya Id

SWINI · Dec 14, 2007

thanks a lot
hey did u watch the topic "design of bot" posted by me
try to reply that please

nan_ishan · Dec 14, 2007

wud u please post the link to the topic

ehsanelahimirza · Dec 14, 2007

nan_ishan said:
ehsanelahimirza said:

MOS is faster then BJTs as i think it for the switching delays.
for BJTs certain amount of current is to b maintained if we have to driver some circuit and that will depend upon the B(beta of BJT)

but for CMOS we can get sharp rise and falls of voltage for its switching as compared to BJT which will take some delay to aquire a value of current

Click to expand...

did u check out d links posted by me

yes i did but there was no thing new in 1st one
and 2nd one yes good but i got only the overview as it is very difficult to read the whole page on screen

nan_ishan · Dec 14, 2007

ehsanelahimirza said:
yes i did but there was no thing new in 1st one
and 2nd one yes good but i got only the overview as it is very difficult to read the whole page on screen

Check the review topic at the end of page in first link...

This line is form second link:Bipolar transistors are used preferably when large load and speed are needed:

fala · Dec 14, 2007

hello nan_ishan, your comparison is not correct. As I said there are many factors that determine bandwith of BJT or MOS. There are fast MOS, slow MOS, fast BJT, slow BJT. each of them are useful for a purpose. If you mean what will be speed limit of these technologies. Well at least I have not seen MOSes that can go beyond 1GHz but ther are BJTs that can operate happily in microwave area.
regards

ehsanelahimirza · Dec 14, 2007

fala said:
There are fast MOS, slow MOS, fast BJT, slow BJT. each of them are useful for a purpose.

thats the basic theme for this topic according to me.

actually MOS or BJTs, selection depends upon type of application and what power involved to drive or to be driven.
motor driving circuit, high VA power supplies/inverters, switching power supplies and memory elements, some times we look for power not the delay but some time exact timings are necessary at low power

and nan_ishan
i think u r reffering to last 2 points given there
again that depends upon application and ur requirements
that for "large load" BJT is used

nan_ishan · Dec 15, 2007

My discussion is about general purpose......
Very Basic MOS andBJts......the very basic technology and not their advanced modified variants

osmansafa · Dec 15, 2007

so you are saying a general purpose bjt is faster than a general purpose mos?

BJT is faster than MOS-Every reply will b clicked helped

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