Distributed amplifier

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The distributed amplifier is an unconventional technique that allows an amplifier designer to escape the tradeoff between gain and bandwidth. With conventional amplifiers, if the gain of one stage is not enough, the designer has to cascade stages. The midband gain of the resulting two-stage amplifier is calculated by simply multiplying the midband gains of each of the stages. However, the bandwidth (3dB cutoff frequency) of the two-stage amplifier is lower than the bandwidth of each of the stages by itself. In most situations the resulting risetime, <math>t_r</math> is closely approximated by <math>t_r = \sqrt{t_{r1}^2 + t_{r2}^2}</math>, where <math>t_{r1}</math> is the risetime of the first amplifier and <math>t_{r2}</math> is the risetime of the second amplifier.

For example:

Amplifier 1:

  • midband gain: 10
  • risetime: 3ns

Amplifier 2:

  • midband gain: 12
  • risetime: 4ns

Cascade of Amplifier 1 and Amplifier 2:

  • midband gain: 120
  • risetime: 5ns

Consider a designer who is working with a technology that produces amplifier stages like Amplifier 1 in the example above. If he needs a total gain of 100 with a risetime of 3ns, he is constrained by the gain-bandwidth tradeoff and is unable to meet both goals simultaneously. The solution is found in the distributed amplifier. In a distributed amplifier several stages are connected together to form, in effect, a transmission line with gain. The gain is the sum (not the product) of the gains of the stages. The bandwidth of a distributed amplifier is the bandwidth of each of the stages. Thus, it is possible to construct an amplifier with a gain of 100 and a risetime of 3ns by using ten instances of Amplifier 1 connected to form a distributed amplifier.