| Both sides previous revisionPrevious revisionNext revision | Previous revision |
| university:courses:engineering_discovery:lab_12 [06 Oct 2016 23:31] – Jonathan Pearson | university:courses:engineering_discovery:lab_12 [03 Jan 2018 19:39] (current) – [Observations and Conclusions] Doug Mercer |
|---|
| ===== Class A NPN Common-Base and Cascode Amplifiers ===== | ===== Class A NPN Common-Base and Cascode Amplifiers ===== |
| {{ analogTV>VideoNumberHere}} | {{ analogTV>5183327709001}} |
| |
| ==== Introduction ==== | ==== Introduction ==== |
| The cascode amplifier is comprised of a common-emitter input stage and a common base output stage. The common-emitter stage provides high input resistance, which is desirable for voltage amplifiers. The common-emitter stage, however, suffers from the Miller effect, which produces a reduction in amplifier bandwidth as the amplifier voltage gain is increased. When the emitter resistor used to bias the common-emitter stage is fully bypassed with a capacitor, its gain is equal to -(collector resistance)/(r<sub>e</sub> of the common-emitter stage). With the common-base stage as the "load" on the common-emitter stage, the collector resistance seen by the common-emitter stage is r<sub>e</sub> of the common-base stage. If the two transistors used in the amplifiers are well-matched, the r<sub>e</sub> of the common-emitter stage will approximately equal r<sub>e</sub> of the common base stage, and the gain of the common-emitter stage will be approximately -1. The common-base amplifier configuration does not suffer from the Miller effect, so it can provide the required voltage gain without any bandwidth penalty. The two amplifiers together, therefore, can provide voltage gain without incurring the bandwidth reduction due to the Miller effect. | The cascode amplifier is comprised of a common-emitter input stage and a common base output stage. The common-emitter stage provides high input resistance, which is desirable for voltage amplifiers. The common-emitter stage, however, suffers from the Miller effect, which produces a reduction in amplifier bandwidth as the amplifier voltage gain is increased. When the emitter resistor used to bias the common-emitter stage is fully bypassed with a capacitor, its gain is equal to -(collector resistance)/(r<sub>e</sub> of the common-emitter stage). With the common-base stage as the "load" on the common-emitter stage, the collector resistance seen by the common-emitter stage is r<sub>e</sub> of the common-base stage. If the two transistors used in the amplifiers are well-matched, the r<sub>e</sub> of the common-emitter stage will approximately equal r<sub>e</sub> of the common base stage, and the gain of the common-emitter stage will be approximately -1. The common-base amplifier configuration does not suffer from the Miller effect, so it can provide the required voltage gain without any bandwidth penalty. The two amplifiers together, therefore, can provide voltage gain without incurring the bandwidth reduction due to the Miller effect. |
| |
| The approximate overall gain of the cascode stage can be quickly determined by inspection, using a few simplifications used in the common-emitter lab. The first simplification is to ignore base currents and think of the emitter and collector currents in both transistors all being equal. Referring to the cascode amplifier schematic, we can think of the same current flowing through the path consisting of the collector and emitter of the common-base transistor into the collector and emitter of the common-emitter stage, much in the same way as the current would flow in a series circuit. Using the results from the common-emitter lab, we can see that when the emitter bias resistor is fully bypassed, the AC voltage gain of the cascode amplifier is simply -R<sub>C</sub>/r<sub>e,CE</sub>. In our circuit, part of the emitter bias resistor is bypassed and part is not. THe two 100 Ω resistors are bypassed with the 220 μF capacitor, leaving the 47 Ω resistor unbypassed. The unbypassed portion of the emitter bias resistor R<sub>E,UBP</sub> is in series with r<sub>e,CE</sub>, so the total emitter resistance used for AC signal gain is now R<sub>E,UBP</sub> + r<sub>e,CE</sub>. The general approximate voltage gain for the cascode amplifier can now be expressed as | The approximate overall gain of the cascode stage can be quickly determined by inspection, using a few simplifications used in the common-emitter lab. The first simplification is to ignore base currents and think of the emitter and collector currents in both transistors all being equal. Referring to the cascode amplifier schematic, we can think of the same current flowing through the path consisting of the collector and emitter of the common-base transistor into the collector and emitter of the common-emitter stage, much in the same way as the current would flow in a series circuit. Using the results from the common-emitter lab, we can see that when the emitter bias resistor is fully bypassed, the AC voltage gain of the cascode amplifier is simply -R<sub>C</sub>/r<sub>e,CE</sub>. In our circuit, part of the emitter bias resistor is bypassed and part is not. THe two 100 Ω resistors are bypassed with the 220 μF capacitor, leaving the 47 Ω resistor unbypassed. The unbypassed portion of the emitter bias resistor R<sub>E,UBP</sub> is in series with r<sub>e,CE</sub>, so the total emitter resistance used for AC signal gain is now r<sub>e,CE</sub> + R<sub>E,UBP</sub>. The general approximate voltage gain for the cascode amplifier can now be expressed as |
| |
| <m>A_V = -R_C/(r_{e,CE} + R_{E,UBP})</m> | <m>A_V = -R_C/(r_{e,CE} + R_{E,UBP})</m> |
| In the cascode amplifier studied in this lab, the collector DC bias current I<sub>C</sub> is approximately 2.8 mA, so r<sub>e,CE</sub> can be calculated to be | In the cascode amplifier studied in this lab, the collector DC bias current I<sub>C</sub> is approximately 2.8 mA, so r<sub>e,CE</sub> can be calculated to be |
| |
| <m>r_e ≈ {26 mV}/{2.8 mA} ≈ 9 Ω</m> | <m>r_{e,CE} ≈ {26 mV}/{2.8 mA} ≈ 9 Ω</m> |
| |
| We can now calculate the overall voltage gain as | We can now calculate the overall voltage gain as |
| The output voltage is biased up at approximately 3.7 VDC which allows a +/-1 V output signal to swing with minimal distortion. | The output voltage is biased up at approximately 3.7 VDC which allows a +/-1 V output signal to swing with minimal distortion. |
| ==== Observations and Conclusions ==== | ==== Observations and Conclusions ==== |
| * Emitter-follower amplifiers provide significant current gain and unity voltage gain | * Common-base amplifiers have low input resistance and are therefore seldom used as voltage amplifiers |
| * The operation of an emitter-follower amplifier involves a form of negative feedback | * Common-base amplifiers are sometimes used as standalone current-in/voltage-out and current buffer amplifiers |
| * Emitter-follower amplifiers have high input resistance and low output resistance | * A very common use for a common-base amplifier is as the output stage of a cascode amplifier |
| * Emitter-follower amplifiers are often used to buffer heavy loads that require large output currents from sources that have high source resistances | * A cascode amplifier is comprised of a common-emitter input stage and a common-base output stage |
| * Adding an emitter-follower stage to a CE amplifier can significantly increase the power gain of the overall amplifier | * The Miller effect occurs in common-emitter amplifiers and causes the bandwidth of the amplifier to decrease as the voltage gain of the amplifier increases |
| | * The load resistance of the common-emitter stage in a cascode amplifier is equal to the low incremental emitter resistance of the common-base stage, and this keeps the magnitude of the voltage gain of the common-emitter stage ≤ 1, minimizing the Miller effect |
| | * Voltage gain in a cascode amplifier is provided by the common-base output stage, which does not suffer from the Miller effect |
| | * The output voltage of a cascode amplifier is out-of-phase with its input voltage |
| | * Voltage gain of the cascode amplifier is similar in form to that of a common-emitter amplifier |
| | * An emitter-follower stage can be added to a cascode amplifier output in order to drive low impedance loads in much the same way that it is done with common-emitter amplifiers |
| | |
| | **Return to [[university:courses:engineering_discovery|Engineering Discovery Index]]** |
