Distributed amplifier: Difference between revisions

Latest revision as of 07:44, 2 October 2024

The distributed amplifier is an unconventional technique that allows an amplifier designer to escape the tradeoff between gain and bandwidth.

Problem

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 (3 dB cutoff frequency) of the two-stage amplifier is lower than the bandwidth of each of the stages by itself. In most situations the resulting rise time, t_r, is closely approximated by

t_r = (t_r1² + t_r2²)^1/2

where t_r1 is the risetime of the first amplifier and t_r2 that of second amplifier.

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

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.

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 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.

History

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, Edward Ginzton, 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 580 series (1959) were the last Tektronix scopes to use distributed amplifiers.

Products

These Tektronix instruments contain distributed amplifiers:

Reading

Distributed Amplifier @ Wikipedia
E. L. Ginzton, W. R. Hewlett, J. H. Jasberg, J. D. Noe, “Distributed Amplification”, Proceedings of the IRE, pp 956-969, August 1948.
John Addis, Good Engineering and Fast Vertical Amplifiers, in Jim Williams (Ed.), Analog Circuit Design: Art, Science and Personalities (1991), p.110
G.Nikandish, R.Staszewski and A.Zhu, The (R)evolution of Distributed Amplifiers: From Vacuum Tubes to Modern CMOS and GaN ICs. IEEE Microwave Magazine Vol. 19 Issue 4, June 2018, p.66+

Patents that may apply to distributed amplifier

Page	Title	Inventors	Filing date	Grant date	Links
Patent GB 460562A	Improvements In and Relating to Thermionic Valve Circuits	W.S.Percival	1935-07-24	1937-01-25	Distributed amplifier • 513 • 514 • 517 • 524 • 541 • 543 • 545 • 551 • 555 • 581 • 585 • 82 • 86 • 945
Patent US 2930986A	Distributed amplifier	John Kobbe • Bill Polits	1956-02-29	1960-03-29	Distributed amplifier • 513 • 514 • 517 • 524 • 541 • 543 • 545 • 551 • 555 • 581 • 585 • 82 • 86 • 945

Distributed vertical amplifier in 545
Distributed vertical amplifier in 555

@@ Line 1: / Line 1: @@
-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.
+==Problem==
 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
+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
-by itself.  In most situations the resulting risetime, <math>t_r</math>
+by itself.  In most situations the resulting rise time, t<sub>r</sub>,
 is closely approximated by
-<math>t_r = \sqrt{t_{r1}^2 + t_{r2}^2}</math>, where
+:t<sub>r</sub> =  (t<sub>r1</sub><sup>2</sup> + t<sub>r2</sub><sup>2</sup>)<sup>1/2</sup>
-<math>t_{r1}</math> is the risetime of the first amplifier
+where t<sub>r1</sub> is the risetime of the first amplifier and t<sub>r2</sub> that of second amplifier.
-and
-<math>t_{r2}</math>
-is the risetime of the second amplifier.
-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.
-Amplifier 1:
+Consider a designer who is working with a technology that produces
-* midband gain: 10
+amplifier stages like the first amplifier in the example above.  If he needs
-* risetime: 3ns
+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.
-Amplifier 2:
+==Solution==
-* midband gain: 12
+[[File:Distributed amplifier principle.png|thumb|400px|right|Distributed amplifier principle]]
-* risetime: 4ns
+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.
-Cascade of Amplifier 1 and Amplifier 2:
+Thus, it is possible to construct an amplifier with a gain of 100 and a rise time of 3 ns
-* midband gain: 120
+by using ten instances of the ×10, 3 ns amplifier from the earlier example connected to form a distributed amplifier.
-* risetime: 5ns
-Consider a designer who is working with a technology that produces
+The key difference between a distributed conventional cascaded-stage amplifier is that in the former,
-amplifier stages like Amplifier 1 in the example above.  If he needs
+the input of  each stage is the original signal, not the output of a previous stage, thus eliminating
-a total gain of 100 with a risetime of 3ns, he is constrained by the
+the cumulative degradation of rise time that occurs in cascaded stages.
-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
+One of the most important challenges when building distributed amplifiers is avoiding reflections in the signal path.
-composed of cascaded stages is that in a distributed amplifier, the input of
+For example, when the input signal reaches the input of one stage, parasitic capacitance of that stage must not
-each stage is the original signal, not the output of a previous stage.  This
+cause an impedance discontinuity in the signal path, which would cause reflection.
-eliminates the cumulative degradation of the risetime that occurs in conventional
+[[File:Tek 581 vertical output amp.png|thumb|right|400px| [[581|Tektronix 581]] distributed vertical amplifier schematic (click to enlarge)]]
-cascaded stages.
+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.
-One of the most important challenges when building distributed amplifiers is
+The design of distributed amplifiers is closely related to the design of lumped-element
-to avoid reflections in the signal path.  For example, when the input signal
+delay lines made from L-C sections.  This, in turn, is based on the notion that a transmission
-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.
@@ Line 66: / Line 54: @@
 energy loss in the transmission line, which limits the number of stages.
-The following Tektronix instruments contain distributed amplifiers:
+==History==
+[[File:Tek513-dist-amp.jpg|thumb|250px|right|Symmetrical distributed amplifier in [[513]] scope]]
+The idea of a distributed amplifier goes back to [[Patent GB 460562A|British Patent 460,562 by W.S. Percival in 1936]].
+In 1948, [[Edward 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]]
 * [[514]]
@@ Line 79: / Line 87: @@
 * [[585]]
 * [[82]]
+* [[86]]
 * [[945]]
+</div>
 == Reading ==
-* [http://w140.com/US2930986.pdf US Patent 2,930,986]: [[John Kobbe|J. R. Kobbe]], "Distributed Amplifier"
+* [[wikipedia:Distributed_amplifier|Distributed Amplifier]] @ Wikipedia
+* E. L. Ginzton, W. R. Hewlett, J. H. Jasberg, J. D. Noe, “Distributed Amplification”, Proceedings of the IRE, pp 956-969, August 1948.
+* [[John Addis]], ''Good Engineering and Fast Vertical Amplifiers'', in Jim Williams (Ed.), ''Analog Circuit Design: Art, Science and Personalities'' (1991),  p.110
+* G.Nikandish, R.Staszewski and A.Zhu, ''[https://hertz.ucd.ie/publications/DA_Review.pdf The (R)evolution of Distributed Amplifiers: From Vacuum Tubes to Modern CMOS and GaN ICs]''. IEEE Microwave Magazine Vol. 19 Issue 4, June 2018, p.66+
+{{PatentLinks|distributed amplifier}}
-[[Category:Circuits]]
+<gallery>
+Tek 545 distributed amplifier on.jpg|Distributed vertical amplifier in [[545]]
+Tek_555_V-Amp.jpg | Distributed vertical amplifier in [[555]]
+</gallery>
+[[Category:Circuits and Concepts]]