Cascode input and VAS amplifier design, will it work :)

[BachAudioDK]

Hi,

12 years ago I build an bipolar version of the Elektor Electronics Cresendo amplifier, and it sounded great. This design used a Cascode VAS, which I think was the main reason for good THD performance for this amp.

Now I have made a new design, which is based on the Cresendo input stage, and Cascode VAS. The new thing I want to try is to use Cascode in the input stage, together with active current mirrors.

Im thinking of using Analog Devices SSM2220 PNP and SSM2212 NPN match transistors for the input stage and the Cascode input stage.

Has anyone experience with the combination of Cascode input stage + current mirrors ?

I have attached a preliminary schematic of my design.
(it may have some errors, as I have not build it yet )

Any feedback is appreaciated !

Regards
Flemming Bach
Denmark

[djoffe]

I believe you won't get a predictable quiescent current in the Q27-Q28-Q29-Q30 stage. You will have two dueling very high gain circuits, for the pull down and pull up...this is a version of the same issue that faces the Randy Slone circuit discussed at length elsewhere on the forum...I fear this is kind of a fatal flaw, without some surgery...

[Workhorse]

Current mirrors at both collector loads of differential will cause instability of idle operating point...................Fatal Mistake.

[alexcp]

Bob Cordell in his recent book, Designing Audio Power Amplifiers, descibes a stabilized current mirror - see the attached schematic - which should work with a complementary differential input stage.
current mirror.JPG
Also, a cascode VAS has a high output impedance, so you need a high input impedance output stage. The one shown in your schematic, a simple emitter follower, would overload your VAS and degrade its performance. If you want to stick with BJTs, you'll probably need a triple emitter follower to keep VAS performance high.

[Lazy Cat]

Because LTP input diferential amp is voltage orientated circuit with high output impedance, it is natural to load it with high impedance interlink amplifying stage in "keep it simple" manner, like mosfet transistor or similar. In this way of solution you have nice transfer from pure voltage to pure current.

[Piersma]

Contributions by Glenn Kleinsmidt and fab.
Please take a look:

Lateral mosfet w Normal or Diamond buffer?

My new mini Class A amp.

regards,
Piersma

[BachAudioDK]

Hi,

Thanks for feedback!

I was also suspicious of the DC behavior of the current mirror in the LTP.

Looking at Bob Cordell´s circuit in figure 8, on page 9:
http://www.cordellaudio.com/papers/MOSFET_Power_Amp.pdf

I have made a new circuit based on Bob´s design.
I think Bob´s explain how it works very well, so I wont repeat it here.

Any comments about using this concept ?

Is the Diodes (D2 and D3) in Bob´s design a good thing to have ?

To Alexcp: Yes you are right, my circuits needs some extra emitter follower so the VAS is not loaded.

BR
Flemming Bach

[BachAudioDK]

Hi,

I found that the Technics SEA3 amplifier, does use a current mirror in the LTP inputstage, which also happens to be connected to a Cascode couplede to the JFET input stage. I dont see that they do anything special to bias/balance the upper current mirror. But again they also do not have any source resistors in the JFET input stage, and they use dual transistors which are properbly pretty equal matched.

@Workhorse: Any comments on the current mirror in the LTP, that you mentioned was fatal ?

But Robert Cordells circuit of how to bias the current mirror does seem to a smart trick

BR
Flemming

[Samuel Groner]

I found that the Technics SEA3 amplifier, does use a current mirror in the LTP inputstage, which also happens to be connected to a Cascode couplede to the JFET input stage. I dont see that they do anything special to bias/balance the upper current mirror.

The problem has nothing to do with the cascode, it is related to the complementary push-pull second stage which needs some sort of bias arrangement. In the schematic you show there is no complementary push-pull but rather a differential second stage (some call this push-pull as well, but its surely not complementary).

Samuel