Electronics - Designing with discrete semiconductors

Created; 12/10/2015, Changed; 04/03/2024, 03/03/2024

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There are many different transistors, and a lot of those are the same dice (component) but selected for different parameters and available in a range of part numbers.  Some types such as Bipolar Junction Transistors with symmetrical Emitter and Collector junctions providing bidirectional high voltage blocking but do not appear to be available now.  Evidently they are made and PNP power transistor are within many low voltage drop regulator IC's to withstand high (automotive standard) reverse voltage.  Designing with transistors can give low-cost solutions or solutions that fit in to small or odd shaped spaces.  You should so more modelling than you would with an IC and there are tools, but I use pencil, paper and calculator or a sheet.

Some of the types not commonly available now are;

• Unijunction transistors, these are used in saw tooth wave oscillators and thyristor trigger circuits.  There is a variant called a programmable unijunction which is a small SCR with an anode gate.  

• Diac which has two connections and is like a triac but without the gate connection but triggers at about 30V. 

• Varicap diodes are still available for radio tuning by voltage control. 

• Tunnel diodes which have negative resistance are used to form very high frequency oscillators. 

• Double gate signal SCR's one gate at the cathode and the other at the anode. 

Some solutions using transistors over a wide temperature and voltage range so require extensive modelling using specific transistor data as opposed to using general rule of thumb rules.  For example, much of the lighting in automobiles must be of a specific colour and intensity over a very wide temperature range and must also fail unambiguously if an LED goes open circuit but not dim, so causing a partial failure.  The circuit fit in a small space.  These low cost, robust applications are the most demanding to model. 

High voltage driver for particle deflection vacuum tube applications; 

The Cossor oscilloscope, manual pictured was probably purchased after 1967, 20MHz, chopped dual trace, with wound delay so that the trigger signal can be seen. 

The Y plate amplifier uses a 50V supply, a 10th of the much older CRT in my homemade oscilloscope.  The circuit has many trimmers and compensation components.  It also uses nuvistors rather than FET transistors where high impedances are required, and uses common-base for the plate drivers.

The schematic (circuit diagram) is, as is typical of the time, somewhat unclear because functions are split, such as the second and subsequent differential amplifier stages split.  This is typical of circuits of the time drawn up by a drawing office.  Availability of CAD tools 20 years later resulted in designers finishing off the circuit diagrams more clearly, similarly if they did the PCB design then the completed design works very much better.  The GE Thyristor book on a previous page is by comparison clearer.

I had considered using a valve amplifier output stage - there was a lot that I did not know at the time, particularly understanding the reading of data in data books.  I had purchased this 1970 edition data book and now see that a pentode valve would have had a tiny fraction of the miller capacitance that any transistor has (<<0.1pF) so the power demanded would have been resolved as well because it would not be necessary to overcome the components capacitance provided I placed the driver very close to the CRT base.  There would be heater power of 2W per valve.  Manufactures data books, even old ones, are invaluable to people starting a hobby.  A pentode valve has two screen grids between the control grid and the anode and reduce ga capacitance by 20-fold I observe from the data book compared to a triode valve's 1-5pF.  So pentode valves would still be a better option in this case where the CRT has such low sensitivity. 

I have criticised the circuit below, then changed the circuit showing some of the steps.

I have introduced snubber networks (R+C) that dampen switching circuits previously, and I have also introduced gate drivers.  The circuit below has such damping, but it is not so plainly shown, but the transistor stage has a limited gain (proportional) and capacitance (integral) those parameters are also dealt with within the op-amp in order to make it unconditionally stable (for gain of 1 or more). 

The circuit and its application will require very good electromagnetic and electrostatic metal screening, as well as distance kept between it and anything that could interfere, such as a power supply.  That is the same care issues that I mentioned for the oscilloscope.

The bandwidth limit occurs when the signal has dropped by 3dB from its regular amplitude, but I have used a point roughly when the signal is just beginning to drop and therefore gives a little margin for manufacturing tolerance, on this page. 

C6, R3 are compensation components that reduce the loop gain and make the circuit more stable but less accurate.

The operational-amplifier is driven to saturation, but it recovers from being driven to saturation without loss of speed, unlike much older bipolar op-amps are known to.  But it is important that the BJTs not be driven to saturation, for the same reason, which will not occur normally in these circuits, provided that the voltage input range is clipped suitably.

Some of the design steps below;

Putting the integrated circuit idea aside and work on an all discrete semiconductor solution;

The circuit above should be located close to the CRT connector, such as part of the connector.  The plates are driven from unsymmetrical voltages, and it would be possible to just use one driver but using two drivers in a bridge configuration would probably be best.  It turns out that the tube being World War two era design is quite insensitive and would need a differential amplifier to meet Valve or transistor rating 300V or so, consequently would also be slow because of the CRT's capacitance.  A more modern tube might be 10th the voltage and perhaps 100x faster, which is the case the best 1960s oscilloscopes were 50MHz. 

Maximum voltage may be; 500V,

Output transistor FMMT495 maximum power; 626mW @ 25'C, but from the graph given 450mW up to 60'C. 

Maximum collector current; 150mA, 500mA pulse.

R1, 2 minimum for transistor maximum power transfer; 150K = (500/2)^2 / 0.45 {(V/2)^2 / W}

Maximum supply voltage due to Vceo at max sustaining V current; 650V  = 450V + 200K * 1mA

Maximum voltage due Vcbo at maximum sustainable V current; 520V = 500V + 200K * 100uA

Time constant due to common base drive; 2uS = 5pF + 9pF [Ccbo + CRT deflection plate] * 150K. 

Time constant due to common base drive; 2uS = 2x 5pF [Ccbo] * 220K. [Emitter followers worst case]

Resistor; 1.7W = 500V^2 / 150K. 

The Emitter resistor should carry between for symmetrical speed; 

For AL-0047-11A - emitter followers; 0 to 5.3mA = 2x 400V/150K 

op-amp E drive; ~3V therefore R should 560R = 3V/5.3mA, It turns out 750R looks best.

Or

For AL0047-08A; 0 to 4mA = 2x 450V/220K 

op-amp E drive; 750R = 3V/4mA, It turns out that 1K looks best. 

Increased transistor power consumption with emitter-follower output;  This estimate based on assumed capacitive loading.

Supply voltage reduced to 400V (calculate with; 450V)

Two transistors turned on at the same time.  5pF Ccob extra charge for the bottom transistor to dissipate joules. J = C.V^2

1uj = 5x10-12 * 450^2

At 150KHz is an extra 150mW, Therefore the circuit operate to 150KHz.

• Using the models (47R), the power probe estimates 50% of 7W for 100nS, giving a maximum frequency of 500KHz.

• Using the models (1K), the power probe estimates 50% of 620mW for 2uS, giving a maximum frequency of 300KHz. 

Resistor; 1W = 450V^2 / 220K. 

3BP1 CRT; x and Y plates D1 to D4 have differing capacitance to the other electrodes up to 9pF.  D1, D2 and D3, D4 have 2pF capacitance to each other.  The tube is rated at 1,500V and 2,000V.  At 2,000V, D1-D2 needs 690V for maximum deflection, D3-D4 needs 490V for maximum deflection. 

But having subsequently found a datasheet for the CRT, as well as being low requiring a high drive voltage to cause deflection it also has more deflection plate capacitance and this must be added to the models.  Also, the design parameters have been changed to suit the circuit options tried, they match the models well.

This circuit gave an unsymmetrical rise and fall times, but that was improved by increasing R5, 7 to 1K to give; 200KHz the input dV/dT rate needs to be limited in order to correct the distortion.  The transistors power rating is easy to work out from Thevenin's half voltage is maximum power transfer law. 

The maximum collector voltage is; 500V * 2M2/ (150K + 2M2) about 450V, which is within Vcbo limit 500V at 100uA.

Other models;

The Cossor oscilloscope manual pictures near the top of this page shows a number of capacitors and resistors necessary to maximise the bandwidth, to do the same as the differential amplifier in this diagram.

This looked very good but when a non-square wave input was tested in the model there turn out to be a lot of distortion;

The transistor's collector capacitance is 5pF or less (Ccbeo).  It appears that the model has used the worst case, so it is possible that all the circuits demonstrated will work better than modelled?

• Use a differential amplifier speeds up the circuit and provides a single ended input.  The circuit is bias off until some drive is required, reducing the saving power.  Likely candidates are; TSH4524, TSH4541, TSH4541-q1, TSH4551, TI did not offer SiMetrix models to test the ICs and comparing Analog Devices, parts they don't all simply drop in and replace each other.  

• Texas Instruments spice models can be extracted from the model in Tina-TI.  You load then view the part's properties, save to * .cir, and you can then load it into your chosen simulator and create a symbol.  You also need to check the pin out.  In this case, TSH4541 worked, but LHS6550 failed to reach the point of displaying waveforms.

• Driving VOCM with feedback was not helpful, so it is driven to 0V.  The value of the emitter resistors the base resistors and all have a bearing when the circuit is driven beyond the transistors thermal limit, the mean voltage level dropping at high speed.  It turned out that this input should be set within the datasheet range 1V of supplies.  With VOCM incorrectly set distortion was introduced that was discovered by testing the model with sine or triangle wave signals. 

The low noise and screening of the oscilloscope. 

Mu-metal is best around the oscilloscope's CRT, but I did not have that, so the distance from the mains' transformers was good.  Also, the very high voltage was created with a voltage multiplier from the 250 Vac transformer, which also had a heater winding for the CRT.  The oscilloscope had mains hum in the brightness course brightness control resolved by adding a cathode resistor and no flyback suppression, which I added.  

High voltage approximately switching/sine wave power supply

AL-0048-01A CADSTAR 18.  Component values are arbitrary, so need to be calculated.

My original circuit probably had a single voltage doubler stage -1,000V, The transformer being a TV EHT/line output transformer with loose coupling and high inductance.  Possibly functions as a saturated core oscillator, with just enough drive to oscillate in a rough sine wave?

If this design were to be revised, a CCFD transformer probably could be used.  The problem with doing that in low production quantities is that you can find an excellent part cheap, but there may be a future time when it is not cheap or available any more.  Small companies just have to live with this issue.  Many but not all big manufacturers are still fairly supportive, though. 

The work on this page suits increasing the CRT voltage to 2,000V (1,800V = +450V and -1,350V, if another multiplier stage were added to the rectifier) to obtain maximum brightness before the spot blooms. 

Higher voltage low-speed drive;

There are some high voltage MOSFET types you may find a low current type, here is one; NDFPD1N150CG, On Semiconductors, N Channel, 100 mA, 1.5 kV, TO220. Note; ID=1mA at 1.2KV.  This transistor would require a lot more power than the BJT transistor solution further below. 

That increased power to over come the higher ID and this larger MOSFETs does not have a much higher reverse capacitance, so the circuit could be faster than the solutions below which use high voltage signal transistors.  If higher power consumption is acceptable, then the totem pole high voltage solution at the lower part of this page would be better replaced with a simple but differential amplifier.

Higher Voltage Amplifier

Conclusion;

The all transistor solution is good, but the integrated circuit solution is better. Non-inverting op-amps one for each side might be the best solution which can be tried using LTspice, Tina TI, or the pay-for version of SiMetrix.  But these circuits do a good job any way but are improved by spending a lot of time on them. 

What I learnt and show in some of my blogs is that it is possible to mislead yourself and make a design that works on paper but depends on conditions being too precisely the similar in the final PCB assembly.  Integrated circuits provide a highly calibrated and trimmed function block, that can eliminates trimming during the manufacturing stage, and can provide the best bandwidth. 

The circuit on the right is quite good, works with single ended input to >300KHz, but is not as good as the differential amplifier IC solution further above.

This circuit is similar to the oscilloscope circuit, works well but requires very low value capacitors for integral control, which seems counterintuitive.  Cheaper than any of the solutions with ICs and is good enough.    

Engineering is not about precision but achieving precision or the desired outcome, good user feel ergonomics from imprecise manufacturing, parts and materials.  Having said that, modern electronics design is more like Lego or Meccano in that the pieces do go together without fitting (cutting and filing).

In the next page, I shall deal with compensation a little more but from a control loop perspective.  In a subsequent page, I shall restate and develop PCB to PCB and other filtering.

• Special transistors - examples of BJT with very low C-E saturation voltage these types are not so commonly available now

Your Web Browser; Right click and select open-a-new-tab to see any of the pictures above full screen.

Limitations of these circuits;

This CRT is best driven from a microcontroller DAC outputs plus digital blanking signal at lower frequency, drawing with the beam similarly to how an X-Y plotter driver works.  This CRT does not have adequate brightness to line scan a picture as a TV screen as a consequence of its comparatively low operating voltage.  It is most likely that the CRT was originally made under shared licensing agreements in about 1944 between WW2 allies.  I bought it new, but it probably was new old war surplus stock? 

The power supply circuit is accurate enough and works fine without integrated circuits, but the deflection plate amplifier is more consistent without trimming during its making by adding op-amps, so that either greater speed and lower power consumption can be achieved probably more cheaply. 

Modelling shortens the development time and with care can be used to create a good design without being lured into a false high confidence in the circuits' performance.  Take care the model does not model all the parts, even though there are models for those parts avalible. 

Discuss this see my; Blog page Electronics 

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