Device matchingIn an all-discrete amplifier with differential inputs and complementary topology, transistor pairs should normally be matched for lowest DC offset and distortion.In the β22, this requirement is alleviated to a large degree by dynamic cascoding and class A operation in every stage, making the amplification transistors operate in a very linear manner. The DC offset adjustment trimpot allows you to manually zero the output DC offset during initial setup. Except for Q1 through Q4, you should be able to build a β22 without matching any other transistors, and the result should still be very good. Nevertheless, for an amplifier of this class, there is a level of satifaction in knowing that it will be performing at its best when the most critical devices are matched. The parameter to match differ according to the device type. For BJTs, the parameter to match is Hfe (DC current gain). For JFETs, the parameter to match is Idss (drain current when Vgs=0). For MOSFETs, the parameter to match is Vgs (the gate-to-source voltage) when the device is on. Pre-matched devicesThe most critical devices which require matching are Q1 through Q4. AMB audio shop currently offers pre-matched 2SK170BL/2SJ74BL JFETs for these positions. This not only saves you the trouble of matching these devices (you need a large number of them to achieve good matches), they also make parts sourcing easier because these devices may be difficult to find in some countries.Which devices to matchThe following is listed in the order of decreasing importance. You do not have to match everything in this list. Just doing the first three items will yield excellent results.
How to match the devicesFor all the following measurements, the 12VDC source could be a lab power supply or a well-charged battery.The meter reading will change as the device warms up, so you should use the reading soon after the power is applied, and be consistent going from one device to another. Use a needle-nose plier to handle the device rather than your fingers, so you don't pre-heat it with your body. If you will be measuring different devices at different times, do so when the ambient temperature is about the same for each so that the results will be comparable. The PNP (or P-channel) device test setup in each case is the same as that for the NPN (or N-channel), except the power supply and meter polarities are reversed. Be sure to use the proper setup for each type or you risk destroying the device. Matching BJTsSome DMMs have a built-in Hfe measurement function. If you have one of these DMMs, the easiest way to test the BJTs would be to use the DMM. Otherwise, use the instructions below. Do not use the DMM Hfe mode on JFETs or MOSFETs.
The above diagram illustrate the setup for measuring a BJT's Hfe. The DMM should be set in DC mA mode (note that most DMMs have a separate probe connection for this mode than voltage or ohms). Since a silicon BJT's Vbe(on) is about 0.65V, the voltage drop across the resistor will be 11.35V. Using the 1.5MΩ (1/8W) resistor as shown will result in a base current of about 7.6µA. The mA reading you get on the meter is the collector current. The Hfe is then the meter reading divided by the base current. For example, if you read 3.8mA on the meter, then the Hfe of the transistor is (3.8mA / 7.6µA) = 500. The value isn't very important, all you need to do is to find devices with similar readings. The 1.5MΩ resistor value was selected for good measurement range with the BC550C/BC560C transistors. For other BJTs with much lower or higher Hfe ratings, the resistor value will need to be changed in order to get a proper collector current range. Matching JFETs
This diagram shows the setup for measuring a JFET's Idss. Just as for measuring a BJT's Hfe, the DMM should be set in DC mA mode. The Idss is read directly on the meter. Matching MOSFETs
The setup for measuring a MOSFET's Vgs is shown above. The 150Ω 5W resistor value was chosen to flow about 50mA through the MOSFET. The DMM should be set to DC volts mode (if the meter is not auto-ranging, set to 10V or 20V range). The Vgs is read directly on the meter. Matching CRDs
CRDs are tested with the illustrated setup above. The DMM should be set in DC mA mode. The If current is read directly on the meter. Main: β22 Main | Prev: Parts list | Next: Instructions |
