I was going through this lecture pdf. If I want to have maximum output resistance which cascode configuration I should use. BJT cascode or Mosfet cascode?
\$\begingroup\$
\$\endgroup\$
3
-
\$\begingroup\$ Do the two devices have the same slope (dIout / dVout), which is Early Voltage for bipolars? \$\endgroup\$– analogsystemsrfCommented Jul 9, 2017 at 22:14
-
\$\begingroup\$ actually I was trying to solve this problem....ibb.co/eY64cv ... \$\endgroup\$– new_eclCommented Jul 10, 2017 at 7:35
-
3\$\begingroup\$ "Cascode" and similar terms are over-used. Show the two circuits you have in mind and what you want them to achieve. Don't get hung up on names, and definitely don't assume everyone interprets the names you use the same, or that there is a single universal definition. \$\endgroup\$– Olin LathropCommented Mar 18, 2018 at 13:28
Add a comment
|
4 Answers
\$\begingroup\$
\$\endgroup\$
5
You want to use MOSFETs if you want maximum output resistance. Draw the small signal model and see for yourself. Try stacking N BJTs vs N MOSFETs to form Cascodes and see the limiting case for Rout. Because of a BJT's rpi, maximum output resistance is limited as you stack more and more stages. For MOSFETs the only practical limit is the voltage rails.
Edit: Clarification for the incorrect downvote
For BJT's forming cascodes by stacking ideal transistors it not that beneficial as the stack grows because the resistance rpi compromises the resistance boost of the cascode structure. So as the BJT cascode stack tends to infinity the output resistance tends to rpi and NOT infinity.
-
\$\begingroup\$ you mean max input resistance? not output resistance \$\endgroup\$– DKNguyenCommented May 30, 2019 at 14:38
-
1\$\begingroup\$ No I mean exactly what I said. Do you understand what I am referring to? For BJT's forming cascodes by stacking ideal transistors it not that beneficial as the stack grows because the resistance rpi compromises the resistance boost of the cascode structure. So as the BJT cascode stack tends to infinity the output resistance tends to rpi and NOT infinity. If you have downvoted, please correct it once you understand my point. \$\endgroup\$– MAMCommented May 31, 2019 at 10:21
-
\$\begingroup\$ It won't let my unvote until the answer is edited somehow \$\endgroup\$– DKNguyenCommented May 31, 2019 at 13:45
-
\$\begingroup\$ Ive added my comment as an edit. thanks \$\endgroup\$– MAMCommented May 31, 2019 at 20:02
-
\$\begingroup\$
\$\endgroup\$
3
Of course you would want to use a BJT because it has a higher intrinsic gain and would give the better (higher) output resistance.
Often you don't have that choice because many designs are made with CMOS processes where (good) BJTs are not available.
-
2\$\begingroup\$ Can you provide explanation why intrinsic gain of BJT is more than MOSFET. I understand that transconductance of BJT is more than FETS. It provides larger gain...But I do not know the difference between MOSFET and BJT output resistance. \$\endgroup\$– new_eclCommented Jul 10, 2017 at 7:33
-
\$\begingroup\$ @new_ecl: Look at the file below. ro = VA/I and compare VA between two devices. global.oup.com/us/companion.websites/fdscontent/uscompanion/us/… \$\endgroup\$– emnhaCommented Mar 18, 2018 at 12:05
-
\$\begingroup\$ Higher output impedance may not be "better". \$\endgroup\$ Commented Mar 18, 2018 at 13:26
\$\begingroup\$
\$\endgroup\$
The pdf you quote is carelessy written, with too many ambiguities and unidentified cicuit references. The problem you are asked to solve could be tackled by using simple three terminal models for the circuit elements. Simple inspection will generate the answer you are looking for.
\$\begingroup\$
\$\endgroup\$
2
You haven't defined what exactly you mean by "cascode" in this context. Generally, this means using two transistors in succession so that the output of the first feeds the input of the second.
If so, it should be clear just from a units check that MOSFETs can't work this way, at least not without additional parts. The input to FETs is voltage, and the output is usually current or resistance. Either way, you can't feed the output of one FET directly into another.
BJTs, however, work on both current in and current out. You can therefore connect multiple devices directly to simulate a single device with larger gain. There are multiple ways of doing this, like the Darlington, and what you may mean by "cascode".
answered Mar 18, 2018 at 13:25
-
\$\begingroup\$ You're thinking of cascade. A cascode is a particular configuration of two transistors where the collector/source of one does not feed the base/gate of the other. \$\endgroup\$ Commented Jul 30, 2018 at 23:38
-
\$\begingroup\$ @imm: The usage of cacode that I've seen refer to a NPN and PNP connected so that the collector of the first drives the base of the second. The emitter of the first is then often tied to the collector of the second. This has the overall effect similar to a Darlington, but with only a single B-E drop. \$\endgroup\$ Commented Jul 31, 2018 at 10:37