High Impedance Buffer and Broadband Amplifier for Digital Freq. Meters

(First published by ) in the Wireless Institute of Australia Amateur Radio magazine, October 1980, last update 14 June 2003.)

AN ARTICLE FOR THE WELL EQUIPPED AMATEUR

With the introduction of synthesized transceivers employing the hetrodyning of several mixer crystals with the VCO output of a PLL system, there has grown the need to measure frequencies at low levels. In the majority of cases, because we are dealing with solid state devices, we have levels that are around the order of 10 dBm or less (0 dBm = 1 mW).

The Impedances around such circuits are not very appropriate for measurement with devices of relatively low impedances, particularly when the circuit impedances can range anywhere between tow hundred and several thousand ohms. Consequently a high gain and high impedance device is required if we are to obtain any measurements and accurate measurements respectively. I am sure that we are all familiar with the operating principle of the GDO, in the same way, loading of any oscillator will cause a resultant shift if frequency.

These two devices, the RF buffer and the broadband amplifier, were primarily designed for the input to the front end of a frequency meter and its prescaler, in particular the EA Digital Frequency Counter. The application was the measurement of a Yaesu FT-901D transceiver because some problems were experienced on the 10m bands.

Those familiar with this transceiver know that the crystals and the VCO cover an approximate frequency range from 15 MHz - 43 MHz. The probe and amplifier were used to obtain measurements over this range with no noticeable shift in the final frequency of the transceiver.

THE HIGH IMPEDANCE BUFFER

Three requirements should be met by the probe:

1. High input impedance - should be greater than 1 M ohm.
2. Low input capacitance - typically less than 10 pF.
3. Wide bandwidth - useful over several octaves.

A JFET was chosen as the active device on the input of the buffer. The JFET was followed by a PNP bipolar tansistor - which is used for impedance transformation. The circuit configuration bootstraps the source resistor to minimize input capacitance.

The FET is a process 50 type with a typical gain of 12 dB at 400 MHz and a noise figure of 4 dB. The quoted input capacitance is 3.5 pf with zero gate to source voltage, although at a Vds of 6.0 volts and a Vgs of -4.0 volts this is significantly improved. A typical device of this process is the MPF102 (although I used a 2N254).

The impedance transforming transistor employed in inverted mode was an AF139, which is a PNP germanium transistor (this was used because if was available in the shack and it has a high Ft. This device is used in TV masthead amplifiers, so it works in the VHF region.) The buffer design is adapted from National Semiconductor's application note (AN32).

The layout is not particularly stringent, although good RF practice should be adopted (keep leads short, particularly around the gate of the JFET.)

The capacitor C1 on the input was included to provide high voltage isolation and should be a good quality high voltage capacitor. If you wish to improve the low frequency you may lower C2 so its impedance is less than 50 ohms at the 3 dB roll off.

Figure 1: High Impedance Buffer

THE BROADBAND AMPLIFIER

National Semiconductor process 43 transistors have been selected because they have a minimum F_t of 600 MHz, some selected devices have F_ts within the GHz region. The process 43 transistors are employed in UHF amplifiers and oscillators with collector currents in the range of 1 - 20 mA. Their h_fe is between 40 and 200. I chose a 2N3563 as the active device for the amplifier.

THE DC BIAS

The DC bias is important, at high currents we achieve greater bandwidth capabilities and better stabilisation of current gain. Looking at the design curve for Constant Gain Bandwidth it was decided to run the transistor with a current of Ic = 10 mA and a voltage of Vce = 7 Volts as a trade-off in this curve and the supply voltage of 9 Volts (from a No. 216 battery).

Using the following DC network and certain assumptions we will derive the values for the resistors:

1. Vc = Vcc - Ic Rc ( Ib + Ibias << Ic)
2. Vb = (R1 + R2) / (R2 Vcc)
3. Vb = Ve + 0.6 [Vbe » 0.6 Volt)
4. Ve = Ic Re [Ie » Ic]

Choosing Ic = 10 mA and Rc = 100 ohms we arrive at R1 = 3.8 kW , R2 = 1 kW and Re = 100 W .

Figure 2: Amplifier DC network

THE RF CONFIGURATION

The key to the bandwidth requirement is to use (RF) negative feedback - which also achieves stabilisation (against positive feedback that can lead to oscillation).

The quoted references in the Ham Radio magazine (now defunct) employ a form of series feedback to achieve gain flatness. The result is constant gain but an unfortunate side-effect is increased input impedance by a factor proportional to the feedback and the beta (h_fe) of the transistor. Since beta can be approximated by the following expression h_fe ~ F_t / f , where f is the operating frequency, the transistor achieves higher gain at lower frequencies. The other form of negative feedback is shunt feedback. This form lowers the input and output impedance as well as stabilising the current gain of the device.

The overall ultimate design employs the application of both forms of feedback; the design parameters are included below:

• Choose Rf = (Zo*Zo) / Re [Zo = 50 W ]
• Choose Gain (dB) = 10 log (Rf / Re)

The circuit employs a balun to match the transistor's output impedance without loading it too much. It also covers a wide frequency range, however, increasing the number of turns will lower the 3 dB roll-off point.

Figure 3: Amplifier AC network.

The final circuit is a combination of the DC and AC networks. I chose components which resulted in a gain of 19 dB ( Rf / Re=79 ) with Re equal to 4.7 ohms and Rf equal to 510 ohms (5k6 in parallel with 560R) .

The performance of this amplifier was measured using a signal generator and an attenuator driving the amplifier into a resistive load.

Since we lived (at the time) in a fringe area for Channel 6 and Channel 8, Lismore, I was able to use weak TV signals and a colour TV set to perform the gain measurements in the VHF region. The amplifier was preceded by a step attenuator 0 - 30 dB. The attenuator was adjusted for colour dropout with and without the amplifier present. This provided a rough estimate of 6 dB gain at 178 MHz and 3 dB gain at 192 MHz

Figure 4: The Amplifier

ACKNOWLEDGEMENTS

A special thanks to my father, VK2ZAD, for the opportunity to use his reference library and test equipment. Thanks also to Nathan VK2DDT for providing me with the original initiative to build the probe and amplifier.

SPECIFICATIONS

REFERENCES

1. Wideband IF Autotransformer, John J. Nagle K4KJ, Ham Radio, November 1976, page 10.
2. Wideband Preamp, Ed Pacyna W1AAZ, Ham Radio, Object 1976, page 61.
3. General Purpose Wideband RF Amp, Randall Rhea WB4KSS, Ham Radio, April 1975, page 58.
4. Linear Application Notes, National Semiconductor National Volume 1 AN32, page 7.
5. Transistors Small Signal Field Effect Power, National Semiconductor.
6. Solid State Design for the Radio Amateur, ARRL 1977.

GLOSSARY

• Amplifier: A device which is used to amplify its input voltage, current or power. Amplifiers are usually active devices and consume power in order to amplify. (There is another form of amplifier constructed from passive devices - this is called a parametric amplifier. Parametric amplifiers are pumped by power at some frequency and amplify small signals at some other frequency.)
• Amps: The Amp (A) is the unit of measure of current, which is an indication of the amount of charge flowing through a conductor per second (after Ampere).
• Beta: the DC current gain of a transistor. Ic » b Ib and Ie » Ic.
• Buffer: An amplifier, usually unity gain, which has high input impedance. Ie the input of a buffer amplifier places a minimal load on the connected circuit. Buffer amplifiers are used to isolate one circuit from another - hence the name.
• Capacitor: A passive device that holds charge. The amount of charge is proportional to the device's capacity or capacitance. The unit of capacitance (C) is the Farad, after Michael Faraday. A capacitor stores energy in an electric field. A capacitor tends to look like a short-circuit as A.C. signals pass through, whilst D.C. signals are stored after an initial large inrush of current.
• Circuit: The connection of passive and active devices to perform some electronic function. Passive devices include: resistors, inductors and capacitors; while active devices include transistors, diodes, FET and integrated circuits etc.
• Current: The measure of the amount of charge moving through a conductor per second. See Amps the units. There are two forms of current, direct current (D.C.) and alternating Current (A.C.). When a D.C. current is present, it always flows in the one direction, from the positive terminal of a circuit to the negative terminal. When an AC current is present, it is oscillating, because the voltage of the terminals reverse every half cycle at the rate called the frequency. AC is very useful because a changing current can be passed though transformers (via magnetic fields).
• dB: The logarithmic measure to the base 10 of power. DB = 10 log P1 / P2 or 20 log V1 / V2.
• dBm: The logarithmic measure of power referenced to 1 mW.
  (0 dBm = 1 mW).
• Electron: An electrically charged sub-atomic particle which orbits the nucleus of all atoms. Various atoms have different numbers of electrons and differing orbital shells. Electrons may be stripped from the outer orbits of atoms by various processes such as radiation (heat, ultra-violet light etc), friction (mechanical heat), by electric fields (a potential difference through some media) and during chemical reactions (batteries) When an atom losses electrons it assumes a positive charge and is said to be ionised - this is an unstable state. (Ions are the basis for chemistry and plasma physics.)
• Farad: A measure of capacitance, the ability to hold electric charge (after Faraday).
• FET: Field Effect Transistor. A FET is a voltage controlled device. It has a gate, drain and source. Small voltage excursions between the gate and source produce large voltage excursions across the drain and source. The source of a FET follows the gate voltage and can drive larger currents (has a lower impedance) so it is often used as a unity buffer in the common source configuration.
• Frequency: The measure of how a system oscillates, formerly cycles per second, but now officially given the units Hetz (Hz). If a voltage oscillates (from positive to negative) 1000 times per second it is said to have a frequency of 1 kHz. The USA power mains frequency is 60 Hz, while the Australian mains frequency is 50 Hz.
• F_t: The cut-off frequency of a transistor is the frequency where the gain is equal to unity. Below the F_t the transistor may be used as an (non-parametric) amplifier.
• Gain: The amount of power or voltage amplification. Unity gain implies that the output signal is equal to the input signal. Gain is often measured in dB. G = 10 log Pout / Pin.
• GDO: Grid Dip Oscillator, a device which when loosely coupled to a tuned circuit, is used to find the resonant frequency of the circuit. Resonance is indicated by the dip in the GDO's output, usually monitored on a meter. The GDO was traditionally constructed using a valve and the indication was had by observing the grid current - hence the name. Even though similar devices are now constructed from FET and Bipolar Transistors they are often still called GDOs.
• H_fe: An alternative nomenclature for beta, the DC current gain of a transistor.
• h_fe: The small signal gain of a transistor - ie the AC current gain.
• I: Symbol used for current.
• Ib: Base current.
• Ibias: Bias current usually provided by resistive dividers.
• Ic: Collector current.
• Ie: Emitter current.
• Impedance: The combined measure of resistance and reactance in ohms. Z = Ö ( R² + X² ).
• Inductance: Inductance (L), measured in Henries (H). An inductor stores energy in a magnetic field and tends to impede the flow of current.
• JFET: Junction Field Effect Transistor. The gate is joined to the PN junction in the FET. Small signals on the gate junction are amplified and available at the drain.
• Ohms: a measure of resistance, the ability to impede the flow of current. (After Ohms.)
• P: The symbol used for power, which is measured in Watts, (after James Watt [steam engines]) . P = V I.
• pF: pico Farads - a small amount of capacitance (10^-12 F).
• Resistance: The measure of the ability to impede the flow of current. When current flows in a resistor power is lost in the form of heat. The power that is lost = I²R.
• Reactance: The apparent resistance of a capacitor or inductor at a specified frequency.
  Z_C = 1 / ( 2p f C ) [f is frequency in Hz, C is capacitance in Farads and Zc is the reactance in ohms]
  Z_L = ( 2p f L ) [ f Hz, L Henries, Z_L is the reactance in ohms.]
• R: Symbol used for a resistor or resistance. R = V / I (ohms law).
• Rb: Base resistor.
• Rc: Collector resistor.
• Re: Emitter resistor.
• Resistor: A device that exhibits resistance. Current passing through a resistance follows ohms law I = V / R. The resistor impedes the current flow and produces heat as a result. The Power lost in the resistor is P = I²R;
• RF: Radio Frequency energy.
• Transformer: A device which can transform an AC potential from one value to another. The transformer is a power transfer device, the output power = the input power - losses. Thus 110 volts AC @ 1 A can be transformed to 11 volts AC @ approximately 10 A.
• Transistor: A contraction of Trans resistance. The transistor has a base, emitter and collector junction. The transistor varies its Collector to Emitter resistance depending on the bias current that flows through the base emitter junction. The transistor amplifies the Base - Emitter current. When operating linearly the collector current is b (Hfe) times the base current.
• Voltage: The measure of electric potential. A current is defined to flow from the positive terminal to the negative terminal. (This is a historical mistake because the charge carrier is an electron, which flows from the negative to the positive terminal.) Voltage is measured in Volts after Voltaire.
• Vbe: Base to Emitter voltage, which is constant to the first order. It depends on the temperature of the Base Emitter junction.
• Ve: Emitter voltage referenced to ground.
• Vc: Collector voltage refer