Cascade amplifier
Encyclopedia
A cascade amplifier is any amplifier constructed from a series of amplifiers, where each amplifier sends its output to the input of the next amplifier in a daisy chain
Daisy chain
Daisy chain may refer to a daisy garland created from daisy flowers, the original meaning and the one from which the following derive by analogy:*Daisy chain *Daisy chain *Daisy chain...

.
A cascade is basically a differential amplifier with one input grounded and the side with the real input has no load. It can also be seen as a common collector
Common collector
In electronics, a common-collector amplifier is one of three basic single-stage bipolar junction transistor amplifier topologies, typically used as a voltage buffer...

 (emitter follower) followed by a common base
Common base
In electronics, a common-base amplifier is one of three basic single-stage bipolar junction transistor amplifier topologies, typically used as a current buffer or voltage amplifier...

. Since the input side has no load there is no gain on that side and the Miller effect
Miller effect
In electronics, the Miller effect accounts for the increase in the equivalent input capacitance of an inverting voltage amplifier due to amplification of the effect of capacitance between the input and output terminals...

 does not come into play. In addition, Vds or Vce stays fairly constant which reduces distortion. Its advantage over the cascode is that it does not require as much voltage headroom. Its disadvantage is since it has two legs it requires twice as much current as a cascode for similar performance.

The complication in calculating the gain of cascaded stages is the non-ideal coupling between stages due to loading. Two cascaded common emitter stages are shown below.
Because the input resistance of the second stage forms a voltage divider with the output resistance of the first stage, the total gain is not the product of the individual (separated) stages.
The total voltage gain can be calculated in either of two ways. First way: the gain of the first stage is calculated including the loading of Ri2. Then the second-stage gain is calculated from the output of the first stage. Because the loading (output divider) was accounted for in the first-stage gain, the second-stage gain input quantity is the Q2 base voltage, VB2 = Vo1.

Second way: the first-stage gain is found by disconnecting the input of the second stage, thereby eliminating output loading. Then the Thevenin-equivalent output of the first stage is connected to the input of the second stage and its gain is calculated, including the input divider formed by the first-stage output resistance and second-stage input resistance. In this case, the first-stage gain output quantity is the Thevenin-equivalent voltage, not the actual collector voltage of the stage-connected amplifier. The second way includes interstage loading as an input divider in the gain of the second stage while the first way includes it as an output divider in the gain of the first stage.

By cascading a CE stage followed by an emitter-follower (CC) stage, a good voltage amplifier results. The CE input resistance is high and CC output resistance is low. The CC contributes no increase in voltage gain but provides a near voltage-source (low resistance) output so that the gain is nearly independent of load resistance. The high input resistance of the CE stage makes the input voltage nearly independent of input-source resistance. Multiple CE stages can be cascaded and CC stages inserted between them to reduce attenuation due to inter-stage loading.

Because of the Friis formulas for noise, most designers put the amplifier with the biggest gain first in the series.




CASCADING OF CE STAGES


CASCADING AMPLIFIERS are used to cascade CE,CB,CC stages.cascading of stages are mainly used to build up a signal's input voltage and current to a usable level.


Among all the configurations CE has the highest power gain and thus it is more popularly used for cascading.for matching purpose,mixture of the three common types are mostly preferred.


for example:
common collector has a high input impedence so it can be used as the first stage.also it has a low output impedence,due to it's low output impedence it can be used for last stage so that it can drive a low resistance load.


VOLTAGE GAIN:

The product of the stage gains will be the overall voltage gain of an amplifier with many stages.Thus the overall gain is the sum of gain and phase shift by each of the stage.


CURRENT GAIN:

The current gain is not obtained as the product of stage gain because in case of current,the output of one stage is not the input for the other stage.
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