What is The cascode amplifier

Chapter 4.8 The cascode amplifier

Lessons In Electric Circuits Volume III – Semiconductors Book
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Lessons In Electric Circuits Volume III – Semiconductors Book

  • 220CHAPTER 4. BIPOLAR JUNCTION TRANSISTORSbase junction, which is subject to change with variations in DC bias voltage. Suffice tosay that the voltage gain of a common-base amplifier can be very high.• The ratio of a transistor’s collector current to emitter current is called α. The α value forany transistor is always less than unity, or in other words, less than 1.4.8The cascode amplifierWhile the C-B (common-base) amplifier is known for wider bandwidth than the C-E (common-emitter) configuration, the low input impedance (10s of Ω) of C-B is a limitation for manyapplications. The solution is to precede the C-B stage by a low gain C-E stage which has mod-erately high input impedance (kΩs). See Figure 229,4.61. The stages are in a cascodeconfiguration,stacked in series, as opposed to cascaded for a standard amplifier chain. See “Capacitor cou-pled three stage common-emitter amplifier” (page 264,255) for a cascade example. The cascodeamplifier configuration has both wide bandwidth and a moderately high input impedance.RLRLViVoViVoRLViVoCommon-baseCommon-emitterCascodeCommonemitterCommonbaseFigure 4.61:The cascode amplifier is combined common-emitter and common-base. This is anAC circuit equivalent with batteries and capacitors replaced by short circuits.The key to understanding the wide bandwidth of the cascode configuration is the Millereffect. The (page 288,279) is the multiplication of the bandwidth robbing collector-base capacitanceby voltage gain Av. This C-B capacitance is smaller than the E-B capacitance. Thus, one wouldthink that the C-B capacitance would have little effect. However, in the C-E configuration,the collector output signal is out of phase with the input at the base. The collector signalcapacitively coupled back opposes the base signal. Moreover, the collector feedback is (1-Av)times larger than the base signal. Keep in mind that Av is a negative number for the invertingC-E amplifier. Thus, the small C-B capacitance appears (1+A|v|) times larger than its actualvalue. This capacitive gain reducing feedback increases with frequency, reducing the highfrequency response of a C-E amplifier.The approximate voltage gain of the C-E amplifier in Figure 230,4.62 is -RL/rEE. The emittercurrent is set to 1.0 mA by biasing. REE= 26mV/IE = 26mV/1.0ma = 26 Ω. Thus, Av = -RL/REE =-4700/26 = -181. The pn2222 datasheet list Ccbo = 8 pF. 290,[5] The miller capacitance is Ccbo(1-Av).Gain Av = -181, negative since it is inverting gain. Cmiller = Ccbo(1-Av) = 8pF(1-(-181)=1456pF