In general though we would never give something this difficult on a test or

In general though we would never give something this

This preview shows page 21 - 24 out of 30 pages.

force this to converge after just 3 or 4 iterations. In general, though, we would never give something this difficult on a test or exam. However, to build useful circuits in real life, you will often need to be able to at least perform basic analysis of such complex circuits.
Image of page 21

Subscribe to view the full document.

241 This last lecture will cover the small signal model for the BJT. As we will see, it shares many similarities with that of the MOSFET, so many of our old techniques still apply. Current Topics: BJT SS Examples The basic process for solving BJT small signal circuits is the same as what we studied for MOSFETs. To begin, we must solve the BJT circuit at DC. Next, we replace capacitors and DC voltage sources with a short circuit, and DC current sources with an open circuit. These steps are part of the general small-signal analysis process, and are identical for the MOSFET and BJT. The last step is to replace the BJT with an equivalent small-signal model. BJT Small Signal Models The basic small-signal model for the BJT is called the hybrid-pi model. Like the default model for the MOSFET, this is the general-purpose replacement for the BJT in SS analysis. You will find it tends to work best for finding open-circuit voltage gain (all cases), and input/output resistance when there is no emitter resistor or common-base connection. The primary difference from the MOSFET model is the addition of the resistor from base to emitter, , and the different formula for . Like the MOSFET, both of these small-signal parameters are determined by the particular bias point (DC current solution) of the circuit: As with the NMOS and PMOS, the above model is valid for the NPN transistor. For the PNP model, it is very important to note that the model is identically the same as to the NPN. There are no changes in any voltage or current polarities. Be careful of this small detail. The governing equations for the parameters of the above model are as follows: Transconductance Base-Emitter Resistance Early Effect | | | | | | As with the MOSFET, the BJT may be modeled with an added parallel resistance caused by what is called the Early effect. Note that the formula for for the BJT actually depends on (while Class Notes BJT Small Signal Analysis
Image of page 22
242 channel-length modulation for the MOSFET did not depend on ). Usually this effect is small compared to the magnitude of , hence the approximation. As with the MOSFET, the values of , , and come from our DC solution of the circuit. The value of would be given if needed. Lastly, the value of is the same thermal voltage from the diode exponential model; at standard temperature. Note that the Early effect is less significant for the BJT at DC, and is commonly ignored. It, however, often has a larger effect than the MOSFET in small signal domain. Lastly, like the MOSFET, the BJT has an alternate model, also called the T-model.
Image of page 23

Subscribe to view the full document.

Image of page 24
  • Spring '17

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern

Ask Expert Tutors You can ask 0 bonus questions You can ask 0 questions (0 expire soon) You can ask 0 questions (will expire )
Answers in as fast as 15 minutes