Corrections to transistor multiplier article published in issue 4/2006
John Fielding, ZS5JF
1. Error in Fig 10 and 11
Following the publication of the transistor multiplier article John noticed that there was an error in Fig 10 and 11. Essentially he made a mistake with the base bias connection. He drew Fig 10 and then modified the file to generate 11 so the error was carried over. The base of TR1 should only connect to the secondary of T1 and not as originally drawn. The corrected Fig 10 and 11 are shown here.
2. Power output of tripler
John has received an enquiry from an Italian reader who had built the 50 to 150MHz tripler shown in Fig 6 & 9. He was unable to obtain the same power output that John had achieved and was puzzled as to why.
John dug out the prototype and ran some tests only to find it gave the power claimed. It was then that John realised what the problem was. Fig 6 & 9 are the same circuit but Fig 9 has the extra test point resistors. These are not the problem. The output matching is where the loss of power occurs. The schematics show a 1pF output coupling capacitor (C7), this assumed the multiplier is feeding another transistor as either a buffer amplifier or a second multiplier. The component values are dimensioned to feed into an impedance of about 500 ohms. If trying to drive 50 ohms there will be a large mismatch. To drive 50 ohms requires a change to the output matching and the capacitor values. The modified version of Fig 6 is shown here. The values of C6 & C7 will depend on the value of L2 and the operating frequency. Normally C7 will require varying in value to give the correct impedance match, this depends on the value of L2 and the dynamic impedance occurring at resonance. The value of 56pF is a good starting point when the inductor is about 100nH with a Q of 100. If the inductor Q is lower then C7 reduces in value and if L2 has a higher Q then C7 increases in value. The matching is a standard "capacitor-tapped" arrangement.
3. Input matching
Some confusion arises about the input drive power measurement. John stated that +7dBm was sufficient to drive the multiplier. This of course is measured from a 50 ohm drive source. The base of the multiplier looks like about 500 ohms, so the input drive needs to be matched from the 50 ohm driver. As long as the input base voltage swing is about 1V p-p or more it will work OK. To drive from a 50 ohm source requires a step-up transformation to obtain the voltage swing. Either a 4:1 or 9:1 transformer or another BJT configured as an amplifier will work. In his prototype the 50MHz crystal oscillator output was about 1V p-p into a high impedance and this drives an emitter follower which then drives the multiplier input. Hence, the input drive is still about 1V p-p.
A signal generator when correctly terminated (eg driving a pure 50 ohm load) delivers the indicated voltage, this is the potential difference (PD) mode. But if the signal generator is operated into an open circuit it delivers an output voltage of twice the PD mode, the electro motive force mode (EMF).
If the multiplier base is driven from a 50 ohm signal generator the drive voltage approaches that of an open circuit mode because the base looks like a relatively high impedance, about 500 ohms. So evaluating the exact drive signal level is very difficult if a signal generator is used as the drive source. The reason this occurs is because signal generators are NOT conjugately matched devices but have an output resistance of 50 ohm, usually furnished by an attenuator network in series with the output and the signal source.