High Impedance Buffer and Broadband
Amplifier for Digital Freq. Meters
(First published by
) in the Wireless Institute of Australia
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
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:
- High input impedance - should be greater than 1 M ohm.
- Low input capacitance - typically less than 10 pF.
- 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
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 Ft
of 600 MHz, some selected devices have Fts
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 hfe is between
40 and 200. I chose a 2N3563 as the active device for the
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:
- Vc = Vcc - Ic Rc ( Ib + Ibias << Ic)
- Vb = (R1 + R2) / (R2 Vcc)
- Vb = Ve + 0.6 [Vbe » 0.6
- 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 (hfe) of the
transistor. Since beta can be approximated by the following
expression hfe ~ Ft / 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
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
A special thanks to my father,
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.
|Gain ~ 0 dB
Input = 10 M ohm || 4 pF
Output <= 50 ohms.
|Gain ~ 19 dB
Input ~ 50 ohms
Output ~ 75 ohms
BW ~ 200 kHz - 50 MHz
- Wideband IF Autotransformer, John J. Nagle K4KJ, Ham
Radio, November 1976, page 10.
- Wideband Preamp, Ed Pacyna W1AAZ, Ham Radio, Object 1976,
- General Purpose Wideband RF Amp, Randall Rhea WB4KSS, Ham
Radio, April 1975, page 58.
- Linear Application Notes, National Semiconductor
National Volume 1 AN32, page 7.
- Transistors Small Signal Field Effect Power, National Semiconductor.
- Solid State Design for the Radio Amateur, ARRL 1977.
- 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
- 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
- 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
- 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
- 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.
- Ft: The cut-off frequency of a transistor is
the frequency where the gain is equal to unity. Below the
Ft the transistor may be used as an
- 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 /
- 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.
- Hfe: An alternative nomenclature for beta, the
DC current gain of a transistor.
- hfe: 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
- 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
- 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
- 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.
ZC = 1 / ( 2p f C )
[f is frequency in Hz, C is capacitance in Farads and Zc
is the reactance in ohms]
ZL = ( 2p
f L ) [ f Hz, L Henries, ZL is the reactance
- R: Symbol used for a resistor or resistance. R = V / I
- 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
- 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
- Ve: Emitter voltage referenced to ground.
- Vc: Collector voltage refer