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

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

- Output transformer (T2 on Charlie's - T1 on this board)
  - 200:50 Ohms = 2:1 turns ratio
  - 10 turns primary (on transistor collector), 5 turns secondary (towards output)

Geometric mean min/max beta at operating current from 2N3904 data sheet
- =sqrt(100*300) = 173

Gain bandwidth product divided by operating frequency
- Assume operating frequency of 10 MHz (my IF is actually at 9 MHz)
- = 300/10 = 30

CE current 10 mA
- V(emitter resistor) = 1/10 Vcc = 1.2V
  - R(emitter resistor) = 1.2V/0.01A = 120 ohms
VCE = 0.7V
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
R2 is 1.9V at 480 uA = 3.9K use 3.3K
R1 sources current to R2 and BE junction
- Voltage = Vcc (12V) - 1.9V = 10.1V
- Current = 480 uA + 48 uA = 528 uA
- R1 = 10.1 / .528 mA = 19K, use 15k
Input resistance - 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 =
  - Beta AC * re = 30*2.6 = 78 ohms - predominates
- All in parallel are 75.8 ohms

T1 50:75.8 ohms
- n = sqrt(Zout/Zin) = sqrt(75.8/50) = 1.23
- 9:11 turns ratio
T2 - different than Charlie's design since my Crystal filters are all 50 ohms in/out
- 250:50 ohms
- n = sqrt(250/50) = 2.23:1
- 11:5 turns