Characteristic recorder(1)

Sophisticated audio technology also means knowledge of the individual electrical properties of the components used. The characteristic recorder presented here can measure all typical discrete semiconductors under PC control and display complete curves as well as typical characteristic values. It uses a power-compensated pulse measurement method, which keeps thermal errors to a minimum.

If you want to operate sophisticated audio technology, you also have to know your components. And not only according to the data sheet, but also according to the special specimen you want to use. Especially for the pairing of components in some assemblies (differential amplifiers, current mirrors, parallel circuits of power transistors ......) one should be able to measure the characteristics to be paired. Using the example of an input differential amplifier with JFET's I would like to explain this briefly. The properties to be matched are the two control characteristics Id = f(Vgs,Vds). In the simplest case (and I haven't seen anything else yet) the Idss (i.e. Id at Vgs=0 and Vds=fixed voltage) and possibly Vth is measured by both transistors. These are two points from a curve map and the matching is a first approximation, but not perfect yet. If one would record the complete characteristic maps with a suitable curve recorder, one would have many more data points available and could significantly improve the matching. E.g. I typically use two 2SK170 with 2500 data points each. After an evaluation over smallest error squares I can be completely sure, that the two transistors are similar in the whole desired working range.

 

KennlinienschreiberArchitecture

Fig. architecture of the characteristic curve recorder

System circuit diagram with power supply and ground balancer

The architecture of the characteristic curve recorder is converted into a concrete hierarchical block diagram. The ground balancer generates an analog ground of 1/2 Vdd.

Kennlinienschreiber

DAC / ADC

For the two DAC channels I used inexpensive (ebay ...) modules with 12bit converters in the first step. These output an output voltage from 0 to VDD. This is filtered via the OP's, buffered and converted to GNDA related to +-2.5V. In the layout an extension to own (16bit) DAC's is already planned.

 

DAC DAC

Instead of the MCP4725 modules, 16Bit DAC modules can also be used. These are based on an LTC1655 and operate with an external reference.

 

ModulDAC LT1655

 

DSC01400

DSC01399

The 4 required ADC channels are also realized with ready-made modules. The 16bit converters are divided into 2 banks in order to carry out 2 conversions in parallel.

ADW ADW

VD Power Stage

The VD power amplifier consists of a high voltage OP LTC2057HV and a power amplifier with 2 complementary Darlingtons. This is sufficient for currents up to 2A and can generate output voltages up to +-25V. The voltage amplification can be switched to x1 or x10 via a relay. At the same time, the scaling of the read-back voltage VDMeasOut is also adapted.

 UD PowerStage UD PowerStage

VG Power Stage

Since I wanted to do without regulated, bipolar current sources for the generation of basic currents, I planned a voltage source with switchable series resistors for the control of gate and base. To determine the current I measure the voltage before and after the resistor. Since in addition the measuring ranges of the two voltage measurements can be switched independently of each other, a bandwidth of 100mA results for bipolar transistors up to some nA for gate leakage currents.

 

UG PowerStage UG PowerStage

 

Is Amp

The current flowing through our test object is measured as voltage drop via a shunt against ground. In order not to have too great an influence on the voltage of the test object, the shunts are selected with extremely low impedance and operated with a maximum 20mV voltage drop. The measuring range can be selected in 4 steps. 2A, 200mA, 20mA, 2mA at full scale with 16bit resolution should be sufficient for all measurement tasks. The low measuring voltage is then amplified via a chopper-stabilized OP x100 to the measuring range of the AD converters.

 

ISMeasAmp ISMeasAmp

 

data sheet

      min typ  max
supply voltage analog  Ub V  18  24  26
supply voltage digital  Ud V  10  12  14
gate/base voltage  Vg V  -12    12
gate/base current mA   -120   120
drain/emitter voltage  Vg V  -20    20
drain/emitter current  Id A  -2    2
           
           

 

Downloads

 

Description  Tool  Link
Circuit diagrams  KiCAD, pdf  Prod01_Schaltplan.pdf
PCB production data  KiCAD, Gerber, Excellon  KlSr_FABDat_01.zip
component placement plan  KiCAD, pdf  Bestückungsplan
part list  KiCAD, txt  Kennlinienschreiber_BOM.txt
firmware for Arduino  Atmel Studio 7  HL-Kennlinienschreiber.elf
software for Windows  Visual Studio 2019  publish.zip
DAC PCB produnction data  KiCAD, Gerber, Excellon ModulDAC_LTC1655 PCB Prod V1.02.zip
     

 

Examples of measurement results

 Z Diode IV Diagram

Z-Diode

v/c diagram

 Z Diode IV Diagram 2

Z-Diode

v/c diagram

 JFET 2SK170 Kennlinienfeld

JFET

2SK170

Characteristics diagram

 JFET 2SK170 Steuerkennlinie

 JFET

2SK170

Control characteristics

 JFET 2SK170 Gateleckstrom

 JFET

2SK170

Gate leakage current

 JFET J111 ohmscher Bereich

JFET

J111

ohmic area

 BIP BD745 Kennlinien

BIP

BD745

Characteristics diagram

 BIP BD745 Stromverstaerkung

BIP

BD745

current gain

 MOSFET IRFP240 Kennlinien

MOSFET

IRFP240

Characteristics diagram

 MOSFET IRFP240 Steuerkennlinien

MOSFET

IRFP240

Control characteristics

 MatchErg

automatic matching

100 x 2SK170

best fit

 JFET 2SK170 Match

Vofs

matched pair 2SK170

       
       

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