Background

I’m a firm believer than sharing knowledge and experience is the best way forward to continue learning yourself. It always pay pack at some point. This time Paul Prinz, a fellow implementer of the 3B7 DHT Preamp using the gyrator PCB, came back with a great suggestion. He found a MOSFET which could do high drain currents, it has high transconductance and most importantly the parasitic capacitances were low even close to the BF862. Hooray, I thought.  We may have a great solution here to use the gyrator load for currents above 25mA and with similar performance to the great BF862. There are some other depletion MOSFETs that can do high currents, however they all have relatively high capacitances and low transconductances when VDS is low, like in the cascoded gyrator circuit. 

The BSH111BK is an enhancement MOSFET, so doesn’t have a “depletion” behaviour like the jFETs. This isn’t a problem as the bias voltage can be set by the reference CCS. 

For comparison, here is a brief summary of the key characteristics of these three devices:

	BF862	BSH111BK	MMBFJ310L
Ptot  (W)	0.3	0.3	0.225
VDSmax (V)	20	55	25
VGS off (V)	-1.2		-4
IDSS (mA)	25	210	60
Gfs (mS)	45	640	18
Ciss (pF)	10	19.1	5
Crss (pF)	1.9	1.5	2.5
Coss (pF)		2.7

There are 2 advantages as well as a freebie from this device:

• Pump up the current: It can do up to 210mA. However is unlikely you will use it with more than 40mA with the valves commonly used. BTW: someone asked me if a parafeed amp could be done with a 300B valve and the gyrator board. Yes, it could be done but what a waste of power!
• Lower the output impedance: with the high transconductance it will provide a lower output impedance, despite being operated at lower VDS which impact the transconductance of any FET. This could be a nice solution when implementing a 4P1L or 10Y preamp stage. 
• Protection Zeners: this device comes with a pair of back to back zeners between gate and source. This reduce the component count in the board, nice freebie! Don’t get confused with the BSH111, which doesn’t have the diodes!

Free Lunch

There is no free lunch am afraid. Firstly the pinout is different to the BF862. Argh, I hate when the clever engineers play with this when designing devices. The lack of standard pinout is a clear way of generating more business! A workaround to this issue is to solder it at 45 degree. Not easy but can be done with a magnifier glass or microscope for SMD soldering:

I thought that the challenge was worth for, so I set myself to solder a pair of boards to test this promising device. 

The circuit 

It’s straight forward as per my 4P1L preamp using the gyrator load. The changes are simply to avoid the Gate-Source protection zeners (but leave the one in for Drain-Source). If you want to use this valve as a driver, you can simply avoid the filament current starvation and run it at 650mA (or 325mA in series filament) and increase the HT. Of course you will have to adjust the filament resistor as needed:

As you can see from the datasheet extract below, the VGS will be somewhere between 1 and 1.8V. In this case for 30mA operation is somewhere about 1.5-1.6V. Only drawback is that a gate stopper can’t be easily added without hacking the PCB, so I decided to omit it and see how stable the circuit was going to be. 

Dial it up!

Building the board is really easy and fast. As you can see below, the BSH111BK is soldered rotated 45 degrees. Not easy to solder this way!

The initial tests were very successful. With the scope and quick frequency response the HF pole was around 2.5MHz. When added the output load below is the automated FR analysis. It’s very similar to the BF862 and showing a great HF performance of this stage:

Now looking at the THD response of the 4P1L stage at nearly 10V, you can see how low the distortion is as well as a nicer spectrum with lower odd harmonics. Nice, this is what I love from the 4P1L when you run it above 25mA:

 

 

The proof is in the pudding

Finally, haven’t listened to this gyrator yet. I know, the proof is in the pudding. However, it’s very promising as it measures so far. Hope to report on this more soon.