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Cascode circuit vs. Active load circuit
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dzseki:
The other day I was reading this ( www.ti.com/lit/pdf/sboa048 ) application bulletin about CRT amplifiers. in this article they mention the cascode circuit as a thing from the past, and they present a „state-of-the art” active load circuit in the form of Philips CR3425 hybrid. This circuit was not exactly unknown to me as I learned about this earlier in the AN-1013 app note too (www.ti.com/lit/an/snva031/snva031.pdf) Section 5.5. The common thing about the two is that they are rather vague about explaining the active load circuit.

To my understanding in the conventional cascode circuit the common base (Q1) part lessen the effect of the Miller capacitence of the lower common emitter transistor (Q2). The bandwidth of the circuit then ultimately relies on the time constant set by R1 and the corresponding stray capacitance around the collector of Q2. Transition times can be improved by emitter bypassing and charge pump to some extent.

Now, with the active load circuit seemingly there are two common emitter transistors tied against each other. They claim better bandwidth on this circuit but I don’t really see why, because for one, this circuit does nothing to eliminate the Miller effect, so what is the trick?

I was not able to find the datasheet of the CR3425 hybrid, but was able to find the OM976/1's which is comparable. I attach it for reference.
Kleinstein:
I think the caskode and active load address 2 different weak points.

The problem addressed with the active load is reducing the power consumption and approximately keeping the standing current constant. With just the resistive load the current through the resistor varies quite a bit and thus a higher power and thus larger transistor has to be used. So the effect on improved BW is due to the possibility to use a smaller transistor.  As an in between step there is bootstrapping of the load. This keeps at least the AC current constant and could with a well behaved signal nearly replace the active load.

Ideally one could combine the active load and cascode.
David Hess:
There is no trick.  There is nothing wrong with the active load which doubles the transconductance (push-pull common emitter output stage) but to make the circuit even faster, both the input transistor and the active load can have cascodes added.

The cascode configuration has a second advantage besides reducing the Miller effect.  It also allows a low voltage high speed input transistor to be used with a higher voltage lower speed cascode transistor gaining the advantages of both.  But these two transistor types would not generally be available in a monolithic IC at the same time which is why hybrid construction with high voltage discrete cascode transistors was common in the fastest video amplifiers.

Oscilloscope z-axis amplifiers used all of the usual active load and cascode configurations found in CRT video amplifiers but among variations, also had another one shown below from the Tektronix 400MHz 7834 and 500MHz 7934 oscilloscopes where the common emitter output stages are replaced with emitter followers and the error amplifier is a 5th transistor.  The emitter follower driving a common base transistor is more commonly known as a differential amplifier although in this case it is made with complementary transistors which is a characteristic shared with the folded cascode.

The older 200MHz 7704A used a configuration like your examples but the common emitter NPN output transistor has no cascode and the PNP active load does as shown below.  This example makes me think that the choice of circuit topology has as much to do with available semiconductors as performance requirements.  A high voltage and fast NPN transistor was available when the 7704A was designed however the cascode configuration was needed for the active load because a suitable high voltage and fast PNP transistor was not.  Just to be sure, I checked the parts and it illustrates this perfectly:

Q41113   151-0221-00   PN4258   6-2&15   PNP 900MHz 12V 3pF Ccb
Q41114   151-0208-00   2N4036   6-3&15   PNP 60MHz 90V 30pF Ccb
Q41115   151-0124-00   2N3501   6-6&12   NPN 150MHz 120V 4.5pF Ccb

Note that the 2N3501 is selected for rb'Cc<50ps (collector base time constant) so the actual Ft is much higher than the datasheet specification.  Add an NPN cascode in there and a faster lower voltage part could be used without selection like with the 7834/7934 example but for whatever reason, Tektronix did it this way.
Sylvi:
Hi

As post-2 says, the cascode extends frequency response by shielding the lower Q from the load, keeping the collector voltage for the lower Q nearly constant so it has effectively wider bandwidth. The output impedance of the upper Q is quite high and it needs a light load to provide maximum bandwidth and signal swing.

Bootstrapping the load on the cascode helps maintain the cascode bandwidth and requires that the upper collector is buffered from the bootstrap point and load.

The "active" load is just a current source, which is a standard feature in amplifier voltage gain stages. As the name implies, it keeps the current through the circuit constant and is the same effect as bootstrapping but without some of the turn-on problems a bootstrap might introduce. A benefit of the current source is that the circuit has full performance right from a very low supply voltage up to the full design value.

In a CRT, there is a need for fast-rising voltage steps and to handle high-frequencies, so anything that reduces the need for high-value compensation caps is good: cascoding, current-sources, bootstrapping.

Note that all current sources are not made equally. The 2-BJT type with local feedback has wider bandwidth than the simple buffered voltage references. The speed can be improved if you add a small base-stop to the current-sense BJT, then add a small cap from its base to the collector of the pass element of the current source. 15pF is typical. The current source can also be cascoded.

I think you would look at it as adding the current source is the best first step to improve the circuit, then add the cascode as the next step UNLESS the circuit already has an EF buffered output with a bootstrapped base resistor, then add the cascode first.
David Hess:

--- Quote from: Sylvi on March 13, 2019, 05:20:11 pm ---I think you would look at it as adding the current source is the best first step to improve the circuit, then add the cascode as the next step UNLESS the circuit already has an EF buffered output with a bootstrapped base resistor, then add the cascode first.
--- End quote ---

And if a transistor with sufficient speed and voltage is not available, the cascode is step zero.
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