Subject: Re: 2n2222 as replacement for MPS5179
From: Michael C. Boatright (ko4wx@mindspring.com)
Date: Fri Feb 27 2004 - 21:24:44 EST
Rod, Rod, are you sure it's Q2 and not Q3? I'm assuming you're talking about the QRP-RF PCB ASSY (RF board). On that board, Q2 is a 2N5109 and Q3 is a 2N5179. If memory serves, I popped the 2N5109 once... You really don't want to replace the 2N5109 with a 2N2222 as Q2 runs hot as it is... Assuming you're talking about replacing a MPS5179 with a 2N2222, from an "absolute maximum ratings" standpoint (see below), you should be just fine--the 2N2222 exceeds the MPS5179 ratings in the most critical parameters. So it probably "won't blow." Most importantly, the 2N2222 has a higher power dissipation (and if its a driver, the Index drives its drivers pretty hard). However, the MPS5179 (2N5179) is significantly lower noise, and its Ft is a good 3 times higher. The minimum Ft of the 2N2222 is 250, according to the Philips datasheet. Using "Good Engineering Practice" (or W1FB, which is, I guess pretty much the same thing), you want the Ft to be 5 times the operating frequency of the amplifier. Q2/Q3 is the IF amplifer, running at the 50MHz IF, so you're right in there with that parameter, and the minimum DC gain (hFE) is actually higher in the 2N2222 (but of course, with a 900MHz fT the 2N5179 is a much better VHF transistor). The biggest consideration is the noise factor. So what may happen is that you may actually raise the overall noise factor of the Index, which for the same bandwidth, will lower the minimum discernable signal--per EMRFD, MDS (dBm) = -174 dBm + NF(dB) + 10 log(B), where B is bandwidth in Hz. So, it'll probably work (and sounds like it does), but if you want the same performance as before, I'd order another 5179 and put it in, since it is in the 50MHz IF. Might be OK, might not. YRMV. An interesting exercise, however, would be to measure the MDS with the 2N2222 in place of the MPS5179. Would be interesting to observe the actual effect. 72 de Mike, KO4WX ABSOLUTE MAXIMUM RATINGS MPS5179 VCBO (Collector-Base Voltage) - 20V VCEO (Collector-Emitter Voltage) - 12V IC (Collector Current - Continuous) - 50mA PD (Total Device Dissipation) - 350mW 2N2222 VCBO (collector-base voltage open emitter) - 60V VCEO (collector-emitter voltage open base) - 30V IC (collector current (DC)) - 800mA PD (total power dissipation) - 500mW ELECTRICAL/SMALL SIGNAL CHARACTERISTICS MPS5179 hFE (DC Current Gain IC = 3.0 mA, VCE = 1.0 V 25) - Min 250 fT (Current Gain - Bandwidth Product IC = 5.0 mA, VCE = 6.0 V, f = 100 MHz) - Min 900MHz; Max 2000 MHz Ccb (Collector-Base Capacitance VCB = 10 V, IE = 0, f = 0.1 to 1.0 MHz) - Max 1.0 pF NF (Noise Figure IC = 1.5 mA, VCE = 6.0 V, RS = 50§Ù, f = 200 MHz) - Min 5.0 dB 2N2222 hFE (DC current gain IC = 10 mA; VCE = 10 V) - Min 75 fT (transition frequency IC = 20 mA; VCE = 20 V; f = 100 MHz) - Min 250MHz Cc (collector capacitance IE = ie = 0; VCB = 10 V; f = 1 MHz) - Max 8 pF NF (noise figure IC = 200 mA; VCE = 5 V; RS = 2 kW; f = 1 kHz; B = 200 Hz 2N2222A) - Max 4 dB Michael C. Boatright
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