Distributed amplifier: Difference between revisions

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Revision as of 23:31, 28 August 2014

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.

The key difference between a distributed amplifier and a conventional amplifier composed of cascaded stages is that in a distributed amplifier, the input of each stage is the original signal, not the output of a previous stage. This eliminates the cumulative degradation of the risetime that occurs in conventional cascaded stages.

One of the most important challenges when building distributed amplifiers is to avoid reflections in the signal path. For example, when the input signal reaches the input of one stage, it is important to avoid having the parasitic capacitance of that stage cause an impedance discontinuity in the signal path, which would cause reflection. Since eliminating the parasitic capacitance is not possible, the approach is usually to reduce the capacitance of the transmission line in the region of an amplifier so that the amplifier's parasitic capacitance can substitute for the capacitance of that region of the transmission line, this avoiding impedance discontinuities. The design of distributed amplifiers is closely related to the design of synthetic delay lines made from L-C sections. This, in turn, is based on the notion that a transmission line can be modeled as a series of L-C sections.

In a properly designed and constructed distributed amplifier, the input signal passes by (and is applied to) the inputs of each of the stages, and is finally absorbed by a terminator, which has the characteristic impedance of the input transmission line of distributed amplifier. In practice, there are several limits that apply to the distributed amplifier technique. One of these is the real component of the input impedance of many amplifying devices. The real component necessarily causes energy loss in the transmission line, which limits the number of stages.

The following Tektronix instruments contain distributed amplifiers: