RF-Amp

RF Amplifier Features

• Useful as an IF Amplifier
• From Charlie Morris' (ZL2CTM) Go QRP Portable SSB Rig
  • Charlie references Solid State Design for the Radio Amateur (pp 19-20)
• Single 2N3904 NPN transistor
  • Ft = 300 MHz (Gain Bandwidth Product)
    • Theoretical gain
      • +20 dB at 30 MHz
      • +29.5 dB at 10 MHz
      • Reality is lower due to capacitance, etc.
• Measured +22 dB gain @12V, +25dB gain @14V
• Input connectors: SMA or BNC
• +12V nominal power
• 49x49mm card
• 4x 4-40 mounting holes

RF Amplifier Design

Schematic

LT Spice Simulation

• LTspice Simulation - GitHub source file
  • +28.4 dB at 9 MHz

Charlie Morris Design

• From Charlie's notes with mods for my use
  • Charlie Morris' (ZL2CTM) Go QRP Portable SSB Rig
  • Charlie describes the design in detail in his video Simple SSB Rig: Part 6 - IF Amplifiers (Feb 2021)
  • Based on the Class A RF Amplifier in Solid State Design for the Radio Amateur pp 19-20
• 2N3904 data sheet
• Emitter Resistance - helpful paper

Beta DC

• Geometric mean min/max beta at operating current
  • =sqrt(100*300) = 173

Beta AC

• Gain bandwidth product divided by operating frequency
  • Assume operating frequency of 9 MHz (IF frequency)
  • = 300/9 = 33.3

DC Operating Point

• Max HFE RF gain at CE current of 10 mA
  • If Vce = 6V, this is 60 mW power dissipation
• Assume Ve (voltage across emitter resistor) = 1/10 Vcc = 12V/10 = 1.2V
  • R3 is Re (emitter resistor) = 1.2V/0.01A = 120 ohms
• VCE = 0.7V (typical from data sheet)
• V(emitter) at 10% of Vcc rule of thumb = 1.2V
• V(base) = V(emitter) + VCE = 1.9V
• Base current is collector current divided by Beta DC
  • Biasing resistors = 10x current needed by base current
  • 10 mA in C-E, beta DC less = 10 mA/173 = 58 uA
  • 10x the current in the biasing resistors = 580 uA (calculated)
• R2 is 1.9V at 580 uA = 3.29K use 3.3K
• R1 sources current to R2 and transistor base
  • Voltage = Vcc (12V) - 1.9V = 10.1V
  • Current = 577 uA + 58 uA = 635 uA
  • R1 = 10.1 / .635 mA = 15.9K, use 15K

Measured DC operating point

• Measured with no input
• Vcc = 11.96V
• Current draw = 12 mA
  • Quick test for wiring and more or less correct parts
  • Expected 11 mA - close enough
• +BUFF = 11.84V
  • 0.12V which is 12 mA through R4 10 ohms - expected
• V emitter = 1.41V
  • 12 mA through 120 ohm = 1.44V - close
• V on input divider = 2.06V
  • Vbase + 0.7V - close

Input resistance

• Xc for 0.1uF cap from emitter to ground
  • C=0.1uF
  • F=10MHz
  • 1/2*pi*F*C = 0.16 ohms
• Parallel resistors R1, R2 paralleled with transistor input impedance
  • R1=15K, R2=3.3K
  • Transistor resistance = Beta AC (33.3) times re
    • re = 26 / Ie (10 mA in mA) = 26/10 = 2.6
    • SSDRA uses 25 as constant - close enough
    • Beta AC * re = 33.3*2.6 = 83.3 ohms - predominates
  • All in parallel are 80.8 ohms

Input/Output Transformers

• FT37-43 Toroid

Tracks

Input Transformer

• Input Transformer (T1 on Charlie's - T2 on this board)
• Need to calculate turns ratio
• 50:80.8 Ohms
• n = sqrt(Zout/Zin)sqrt(80.8/50) = 1.27 turns ratio
• Turns choices
• Minimum number of turns
• Rule of thumb - want Xl (coil impedance smallest value) to be least 4-5X the load
  • Load = 80.8 ohms
  • 5 * 80.8 ohms = 404.2 ohms minimum
    • More turns = larger capacitance and drops bandwidth
  • Toroid is FT37-43
  • From Toroid page
    • Xl = 404.4 at 9 MHz is 4.5 turns, round up to 5
  • Try nearest integer numbers turns ratios
    • 5:6 = 6% error
    • 6:8 = -4.6%
    • 7:9 = -1.1% << good choice
    • 8:10 = +1.7%
    • 9:11 = +4.0%
    • 10:13 = -2.19%
• Use 7:9 turns ratio for optimal input transformer

Output Transformer

• Output transformer (T2 on Charlie's - T1 on this board)
• T2 - different than Charlie's design since my Crystal filters are all 50 ohms in/out
• SSDRA suggest presenting 200 ohm load to the collector
  • Can't find reference in SSDRA
  • Reflecting back 50 ohms load to 200 ohm collector...
• 200:50 ohms
• n = sqrt(200/50) = 2.0:1 turns ratio
• 10:5 turns
  • 10 turns primary (on transistor collector)
    • 10 turns = 35 uH
  • 5 turns secondary (towards output)
    • 5 turns = 8.75 uH
  • 15 turns = 9.5 in

Charlie's Notes

NanoVNA Measurements

• Goal: Measure RF-Amp performance using a NanoVNA running NanoSaver software on PC
• S21 (gain) needs to be measured with a 40 dB attenuator on input to RF-Amp to avoid compression on the output
• S11 (reflection) input impedance can't be measured with input 40 dB attenuator because S11 just ends up measuring the attenuator
  • Output should be terminated to 50 ohms for S11 measurement
• DC current = 12 mA

Measure S21

• Put 40 dB attenuator on RF-Amp input, measure S21 at output
  • NanoVNA provides 50 ohm load to RF-Amp to properly terminate output
• Measure S21 with 9:11 input transformer
  • S21 @ 100 KHz = -8 dB dB
  • S21 @ 1.45 MHz = 35.4 dB (peak gain)
  • S21 @ 9.1 MHz = 24.3 dB
  • S21 @ 16 MHz = 20.1 dB
  • S21 @ 30 MHz = 12.7 dB
• Peak gain justifies use of 40 dB attenuator to protect NanoVNA

LTspice vs NanoVNA

• LTspice simulation was pretty similar to NanoVNA results
  • -10 dB at 100 KHz
  • +32 dB at peak
  • Lower output at higher frequencies

Measure Input Compression

• Is there compression if the NanoVNA drives the input directly?
  • Test by driving directly from NanoVNA set to CW = 9 MHz
  • Measured output with scope - not clipped at 9 MHz
    • Approx. 1Vpp input = +22.1 dBm gain which matches the S21 with the attenuator on the input
    • Vpp = 12.4V with 50 Ohm load resistor
  • Starts clipping at 7 Mhz and down
• Therefore, can measure input impedance at 9 MHz
• Other evidence of compression
  • Compare S21 gain with no input attenuator, put external 40 dB RF Attenuators on output of RF-Amp to protect NanoVNA input
  • S21 shows lower gain in lower frequencies so clipping/compression is happening
  • Was: 35 dB at 1.4 MHz
  • Is: 23.1 dB at 1.5 MHz
• Due to compression can't accurately measure lower frequencies with attenuator at output
• Compression below 7 MHz matches what was on scope

Measure Input Impedance

• Shows VSWR at 14.4 MHz = 1.56:1
• At 9 MHz
  • VSWR = 1.7:1
  • Impedance = 81-j10

Change Input Transformer turns ratio

• Above had 9:11 turns ratio
• Change to 7:9 turns ratio
• Slightly better gain at higher frequencies
• Was: S21 @ 30 MHz = 12.7 dB
• After: S21 @ 30 MHz = 15.3 dB
• Small additional gain at 8 MHz
  • Was: S21 @ 9.1 MHz = 24.3 dB
  • After: S21 @ 9.1 MHz = 24.8 dB

• New turns improved the input VSWR slightly
• Was: At 9 MHz, VSWR = 1.7:1, Impedance = 81-j10
• After: At 9 MHz, VSWR = 1.6:1, Impedance = 76.7-j12

Tune input transformer

• Isolate output by replacing output transformer with 200 resistor
• Add one more output winding to input transformer T2 (7:10)
• VSWR nearly 1.04:1 at 11.1 MHz
• -19 dB return loss at 9 MHz VSWR = 1.249:1

• With output transformer
• Slightly better with 1 extra winding

Assembly Sheet

• RF Amplifier Assembly Sheet