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The distributed amplifier is an unconventional technique that allows | The '''distributed amplifier''' is an unconventional technique that allows | ||
an amplifier designer to escape the tradeoff between gain and bandwidth. | an amplifier designer to escape the tradeoff between gain and bandwidth. | ||
==Problem== | |||
With conventional amplifiers, if the gain of one stage is not enough, | With conventional amplifiers, if the gain of one stage is not enough, | ||
the designer has to cascade stages. The midband gain of the resulting | the designer has to cascade stages. The midband gain of the resulting | ||
two-stage amplifier is calculated by simply multiplying the midband gains | two-stage amplifier is calculated by simply multiplying the midband gains | ||
of each of the stages. However, the bandwidth ( | of each of the stages. However, the bandwidth (3 dB cutoff frequency) of | ||
the two-stage amplifier is lower than the bandwidth of each of the stages | the two-stage amplifier is lower than the bandwidth of each of the stages | ||
by itself. In most situations the resulting | by itself. In most situations the resulting rise time, t<sub>r</sub>, | ||
is closely approximated by | is closely approximated by | ||
< | :t<sub>r</sub> = (t<sub>r1</sub><sup>2</sup> + t<sub>r2</sub><sup>2</sup>)<sup>1/2</sup> | ||
< | where t<sub>r1</sub> is the risetime of the first amplifier and t<sub>r2</sub> that of second amplifier. | ||
and | |||
< | |||
For example | For example, cascading two amplifiers having a gain of 10 and a rise time of 3 ns, and a gain of 12 | ||
and a rise time of 4 ns, respectively, will result in a mid-band gain of 120 and a rise time of 5 ns. | |||
Consider a designer who is working with a technology that produces | |||
amplifier stages like the first amplifier in the example above. If he needs | |||
a total gain of 100 with a rise time of 3 ns, he/she is constrained by the | |||
gain-bandwidth trade-off and is unable to meet both goals simultaneously. | |||
==Solution== | |||
[[File:Distributed amplifier principle.jpg|thumb|400px|right|Distributed amplifier principle]] | |||
In a distributed amplifier, several stages are connected together to form what in effect | |||
is a "transmission line with gain". The gain is the sum (not the product) | |||
of the gains of the stages, whereas 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 rise time of 3 ns | |||
by using ten instances of the ×10, 3 ns amplifier from the earlier example connected to | |||
form a distributed amplifier. | |||
The key difference between a distributed conventional cascaded-stage amplifier is that | |||
in the former, the input of each stage is the original signal, not the output of a | |||
previous stage, thus eliminating the cumulative degradation of rise time that occurs in | |||
cascaded stages. | |||
One of the most important challenges when building distributed amplifiers is avoiding | |||
reflections in the signal path. For example, when the input signal reaches the input | |||
of one stage, parasitic capacitance of that stage must not cause an impedance discontinuity | |||
in the signal path, which would cause reflection. | |||
[[File:Tek 581 vertical output amp.png|thumb|right|400px| [[581|Tektronix 581]] distributed vertical amplifier schematic (click to enlarge)]] | |||
Since eliminating the parasitic capacitance is not possible, the approach is usually to | |||
reduce the capacitance of the transmission line around each amplifier input (thereby | |||
increasing its impedance) so that the amplifier's parasitic capacitance can substitute | |||
for the capacitance of that region of the transmission line. | |||
The design of distributed amplifiers is closely related to the design of lumped-element | |||
delay lines made from L-C sections. This, in turn, is based on the notion that a transmission | |||
distributed amplifiers is closely related to the design of | |||
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. | line can be modeled as a series of L-C sections. | ||
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energy loss in the transmission line, which limits the number of stages. | energy loss in the transmission line, which limits the number of stages. | ||
The | ==History== | ||
[[File:Tek513-dist-amp.jpg|thumb|250px|right|Symmetrical distributed amplifier in [[513]] scope]] | |||
The idea of a distributed amplifier goes back to British Patent 460,562 by W.S. Percival in 1936. | |||
In 1948, Ginzton, [[Bill Hewlett|Hewlett]], Jasberg and Noe published a paper on distributed amplifiers in the Proceedings of the IRE, first using the term "distributed amplifier". Around the same time, Hewlett met [[Logan Belleville]] of Tektronix in a Portland restaurant and described the concept on a napkin. In the fall of 1948, [[Howard Vollum]] and [[Dick Rhiger]] built a 6 ns rise time distributed amplifier under a US government contract (for radar applications). The prototype was attached externally to an early [[511]] oscilloscope. | |||
Vollum, Belleville and Rhiger went on to design the 50 MHz [[517]] oscilloscope incorporating a distributed vertical amplifier. | |||
The [[581|580 series]] (1959) were the last Tektronix scopes to use distributed amplifiers. | |||
==See also== | |||
* [[Distributed deflection plates]] | |||
==Products== | |||
[[File:585a_dist_vert_amp.jpg|thumb|250px|right|Second distributed amplifier in [[585A]] scope]] | |||
These Tektronix instruments contain distributed amplifiers: | |||
<div style="column-count:8;-moz-column-count:8;-webkit-column-count:8"> | |||
* [[513]] | * [[513]] | ||
* [[514]] | * [[514]] | ||
* [[517]] | * [[517]] | ||
* [[524]] | |||
* [[541]] | * [[541]] | ||
* [[543]] | * [[543]] | ||
Line 78: | Line 89: | ||
* [[585]] | * [[585]] | ||
* [[82]] | * [[82]] | ||
* [[86]] | |||
* [[945]] | |||
</div> | |||
== Reading == | |||
* W. S. Percival, British Patent Specification No. 460,562, ''Improvements In and Relating to Thermionic Valve Circuits'', applied for, July 24, 1936, granted January 1937 | |||
* E. L. Ginzton, W. R. Hewlett, J. H. Jasberg, J. D. Noe, “Distributed Amplification”, Proceedings of the IRE, pp 956- 969, August 1948. | |||
* [http://w140.com/US2930986.pdf US Patent 2,930,986]: [[John Kobbe|J. R. Kobbe]] and [[Bill Polits|W. J. Polits]], "Distributed Amplifier". Applied 29 Feb 1956, granted 29 Mar 1960. | |||
* [http://en.wikipedia.org/wiki/Distributed_amplifier Wikipedia: Distributed Amplifier] | |||
* [[John Addis]], ''Good Engineering and Fast Vertical Amplifiers'', in Jim Williams (Ed.), ''Analog Circuit Design: Art, Science and Personalities'' (1991), p.110 | |||
<gallery> | |||
Tek 545 distributed amplifier on.jpg|Distributed vertical amplifier in [[545]] | |||
</gallery> | |||
[[Category:Circuits and Concepts]] |