10/2/2018 Indian Institute of Technology Jodhpur, Year 2018 Analog Electronics (Course Code: EE314) Lecture 23: Cascode Stage Course Instructor: Shree Prakash Tiwari Email: [email protected] Webpage: http: //home.iitj.ac.in/ ~sptiwari/ Course related documents will be uploaded on http://home.iitj.ac.in/~sptiwari/EE314/ Note: The information provided in the slides are taken form text books for microelectronics (including Sedra & Smith, B. Razavi), and various other resources from internet, for teaching/academic use only 1 Ideal Current Source Circuit Symbol I-V Characteristic Equivalent Circuit • An ideal current source has infinite output impedance. How can we increase the output impedance of a BJT that is used as a current source? 1 10/2/2018 Boosting the Output Impedance • Recall that emitter degeneration boosts the impedance seen looking into the collector. – This improves the gain of the CE or CB amplifier. However, headroom is reduced. Rout 1 g m RE || r rO RE || r Cascode Stage • In order to relax the trade‐off between output impedance and voltage headroom, we can use a transistor instead of a degeneration resistor: Rout [1 g m (rO2 || r1 )]rO1 rO2 || r1 Rout gm1rO1 rO2 || r1 IC 2 I E1 IC1 if 1 1 • VCE for Q2 can be as low as ~0.4V (“soft saturation”) 2 10/2/2018 Maximum Cascode Output Impedance • The maximum output impedance of a cascode is limited by r1. If rO2 r1 : R g r r r out,max m1 O1 1 1 O1 PNP Cascode Stage Rout [1 gm1(rO2 || r1)]rO1 rO2 || r1 Rout gm1rO1rO2 || r1 3 10/2/2018 False Cascodes • When the emitter of Q1 is connected to the emitter of Q2, it’s not a cascode since Q2 is a diode‐connected device instead of a current source. 1 1 Rout 1 g m1 || rO 2 || r 1 rO1 || rO 2 || r 1 g m 2 g m 2 g m1 1 Rout 1 rO1 2rO1 g m 2 g m 2 Short‐Circuit Transconductance • The short‐circuit transconductance of a circuit is a measure of its strength in converting an input voltage signal into an output current signal. iout Gm vin vout 0 4 10/2/2018 Voltage Gain of a Linear Circuit • By representing a linear circuit with its Norton equivalent, the relationship between Vout and Vin can be expressed by the product of Gm and Rout. Norton Equivalent Circuit Computation of vout iout Rout Gm vin Rout short-circuit output current: vout vin Gm Rout Example: Determination of Voltage Gain Determination of Gm Determination of Rout iout vx Gm gm1 Rout ro1 vin ix vout 0 Av gm1rO1 5 10/2/2018 Comparison of CE and Cascode Stages • Since the output impedance of the cascode is higher than that of a CE stage, its voltage gain is also higher. vout gm1vinrO1 VA Av gm1rO1 Av gm1rO2 gm2 rO1 r 2 VT Voltage Gain of Cascode Amplifier • Since rO is much larger than 1/gm, most of IC,Q1 flows into diode‐connected Q2. Using Rout as before, AV is easily calculated. iout gm1vin Gm gm1 Av Gm Rout g m11 g m2 rO1 || r 2 rO 2 rO1 || r 2 g m1g m2 rO1 || r 2 rO 2 6 10/2/2018 Practical Cascode Stage • No current source is ideal; the output impedance is finite. Rout rO3 || gm2rO2 (rO1 || r 2 ) Improved Cascode Stage • In order to preserve the high output impedance, a cascode PNP current source is used. Rout gm3rO3 (rO4 || r 3 )|| gm2rO2 (rO1 || r 2 ) Av gm1Rout 7 10/2/2018 NMOS Cascode Stage Rout 1 gm1rO1 rO2 rO1 Rout gm1rO1rO2 PMOS Cascode Stage Rout 1 gm1rO1 rO2 rO1 Rout gm1rO1rO2 8 10/2/2018 Another Interpretation of MOS Cascode • Similar to its bipolar counterpp,art, MOS cascode can be thought of as stacking a transistor on top of a current source. • Unlike bipolar cascode, the output impedance is not limited by . 17 Example: Parasitic Resistance Rout (1 gm1rO2 )(rO1 || RP ) rO2 • RP will lower the output impedance, since its parallel combination with rO1 will always be lower than rO1. 18 9 10/2/2018 Transconductance Example Gm gm1 MOS Cascode Amplifier Av Gm Rout Av gm1(1 gm2rO2 )rO1 rO2 Av gm1rO1gm2rO2 10 10/2/2018 PMOS Cascode Current Source as Load • A large load impedance can be achieved by using a PMOS cascode current source. RoN g m 2 rO 2 rO1 RoP g m3rO 3rO 4 Rout RoN || RoP Review: Cascode Stage Rout • The impedance seen looking into the collector can be boosted significantly by using a BJT for emitter degeneration, with a relatively small reduction in headroom. Rout [1 g m (rO2 || r1 )]rO1 rO2 || r1 Rout gm1rO1 rO2 || r1 11 10/2/2018 Another View of a Cascode Stage • Instead of considering a cascode as Q2 degenerating Q1, we can also think of it as Q1 stacked on top of Q2 (current source) to boost Q2’s output impedance. Temperature and Supply‐Voltage Dependence of Bias Current • Circuits should be designed to operate properly over a range of supply voltages and temperatures. • For the biasing scheme shown below, I1 depends on the temperature as well as the supply voltage, since VT and IS depend on temperature. VBE /VT I1 I S e R2 VBE VCC R1 R2 12 10/2/2018 Concept of a Current Mirror • Circuit designs to provide a supply‐ and temperature‐ independent current exist, but require many transistors to implement. “golden current source” • A current mirror is used to replicate the current from a “golden current source” to other locations. 13.