RF-Amp

RF Amplifier Features

• From Charlie Morris' (ZL2CTM) Go QRP Portable SSB Rig
  • Solid State Design for the Radio Amateur?
• +22 dB gain
• Input connectors: SMA or BNC
• 49x49mm card
• 4x 4-40 mounting holes

RF Amplifier Design

Schematic

LT Spice Simulation

• LTspice Simulation - GitHub source file

Transformers

Charlie Morris Design

• From Charlie's notes with mods for my use
  • Charlie Morris' (ZL2CTM) Go QRP Portable SSB Rig
• 2N3904 data sheet
• Emitter Resistance

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 10 MHz (my IF is actually at 9 MHz)
  • = 300/10 = 30

DC Operating Point

• CE current 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) = 1.2V
• V(base) = V(emitter) + VCE = 1.9V
• Assume current in biasing resistors = 10x current needed by DC beta
  • 10 mA in C-E, beta DC less = 10 mA/173 = 48 uA
  • 10x the current in the biasing resistors = 480 uA (calculated)
• R2 is 1.9V at 480 uA = 3.9K use 3.3K
  • Actual current will be 1.9V/3.3 ohms = 634 mA
• R1 sources current to R2 and transistor base
  • Voltage = Vcc (12V) - 1.9V = 10.1V
  • Current = 576 uA + 57 uA = 634 uA
  • R1 = 10.1 / .634 mA = 15.9K, use 15k

Input resistance

• Xc for 0.1uF cap from emitter to ground
  • 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 (30) times re
    • re = 26 / Ie (10 mA in mA) = 26/10 = 2.6
    • Beta AC * re = 30*2.6 = 78 ohms - predominates
  • All in parallel are 75.8 ohms

Input/Output Transformers

FT37-43 Toroid

• FT37-43
• Wideband Transformers 5 - 400 MHz
• Power Transformers 0.5 - 30 MHz
• 10 turns = 35uH

Tracks

Input Transformer

• Input Transformer (T1 on Charlie's - T2 on this board)
• 50:75.8 Ohms = 1 : 1.23 turns ratio
• n = sqrt(Zout/Zin) = sqrt(75.8/50) = 1.23
• 9 turns primary, 11 turns on secondary

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
• 200:50 ohms
• n = sqrt(200/50) = 2:1
• 10:5 turns
• 10 turns primary (on transistor collector), 5 turns secondary (towards output)

Charlie's Notes

Assembly Sheet

• RF Amplifier Assembly Sheet