cascode output impedance? - Page 1
Forum section: electronics / projects

author: ridgerunnersjw
author_rank: Contributor
timestamp: August 28, 2022, 01:19:30 pm
content: I am investigating a cascode BJT ckt and everything I see suggests a high output impedance....Is this true? What is the benefit of a high voltage gain amplifier having a high output impedance??Thanks

author: TimFox
author_rank: Super Contributor
timestamp: August 28, 2022, 01:58:57 pm
content: Yes, a cascode (by itself) has a higher output impedance than a simple CE stage.The output impedance is basically that of a CB stage with a high impedance in the emitter circuit.In practice, the impedance at the output will be dominated by the load at the second collector.One advantage of a cascode is reduced feedback from the C of the second transistor to the B of the first transistor.A cascode is often followed by a CC (follower) stage to reduce the impedance driving the next stage.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 28, 2022, 02:52:55 pm
content: Right, the high impedance (bottom collector) into the top emitter more or less cancels out Early effect.For AF purposes, you can use a CCS (which itself can be cascoded) to massively increase DC voltage gain, and a follower to help maintain that gain somewhat independent of load. Note that gain falls at AC, due to node capacitance. Especially when additional capacitance is required for compensation purposes.For RF purposes, you can use for load, a choke with impedance much higher than the load resistance, or resonant tank with lower characteristic impedance (for band limiting), or filter with matched impedance (for more precise band limiting). This can include impedance transformation (matching network or transformer) so that the impedance at the collector is very high, maximizing power gain (which goes as sqrt(Z) versus a CC source, so it's not a huge advantage, but it's something).In any case, note that the collector capacitance is unavoidable, forcing an upper limit (on order of Ccb || R_L), regardless of CCS or choke load. Or even filters. You must design any filter (whether LC tank or more complex designs) to use Ccb as [or part of] the first (shunt) element. In general, this limits GBW, regardless of center frequency or filter order: you might have a stage that's, whatever, 100 gain at DC-1MHz (baseband / wideband), or 100 gain at 9.5-10.5MHz (narrowband / tuned around a center frequency). Higher filter order (which for wideband circuits, is simply called "peaking", but narrowband might simply involve higher order filters of whatever type) does extend GBW, but only slightly (say 30-70%)*.*I forget what the asymptotic (order --> inf) maximum is; in any case, the returns are quickly diminishing past 2nd order or so.*There is a special case for a capacitive load, as for electric deflection oscilloscopes, or CRT video amps. In this case, bandwidth to the C-load can be almost 200% more. I'm not aware of any more than modest improvements for matched loads, however.Tim

author: mawyatt
author_rank: Super Contributor
timestamp: August 28, 2022, 04:13:30 pm
content: A technique we employed decades ago for Ultra-Broadband use (SiGe Amps) was to add a single component to the classic BJT Cascode stage, this increased the BW without additional power consumption. Adding a small inductor between the bottom transistor collector and top transistor emitter creates a "Pi" LC network between the bottom and top, the added L, and the junction capacitance C. The impedance "Seen" looking into the bottom collector and into top emitter are better matched utilizing the single series inductor which creates a somewhat "Pi" lumped element model for a transmission line between the collect and emitter.This technique works very well and is "free" power-wise!!Best,

author: TimFox
author_rank: Super Contributor
timestamp: August 28, 2022, 04:27:01 pm
content: That technique was also used in vacuum-tube cascodes, such as two 6CW4 triodes, placing an inductor between plate 1 and cathode 2.This was a common low-noise amplifier circuit in the 100 to 200 MHz range.For example, see fig 16.3B on p 401 of the 1971 ARRL "Radio Amateur's Handbook".

author: TimFox
author_rank: Super Contributor
timestamp: August 28, 2022, 07:35:02 pm
content: Quote from: T3sl4co1l on August 28, 2022, 02:52:55 pmI used this technique in a discrete-transistor RIAA amplifier to reduce a nasty by-product of the Early effect.There were two PNPs in cascode, where the base of the second transistor was capacitively coupled to the emitter of the first transistor, and the overall RIAA feedback went to the first emitter in the normal manner.Without such a "bootstrapped" CB on top of the first transistor, the common-mode input AC voltage at the first base would induce a real change in the AC component of the base current (due to change in the BC voltage), which would flow through the substantial source impedance, a normal MM phono cartridge.The voltage induced thereby at the input would be indistinguishable from the input signal, and the overall negative feedback would not compensate for it.