philips: Philips AE 3905: Unorthodox amplification circuits design ?

This radio model uses 8 x MM74HCU04M inverter IC's for a total of 48 CMOS gates biased as analog amplifiers. Kind of unorthodox amplification circuits design, don't you think?

Integrated circuits using complementary metal oxide semiconductor (CMOS) gates were designed for binary operation working as two state switching devices in digital circuits, also adequate to work as crystal or RC oscillators and Schmitt Triggers circuits.

However, several unbuffered inverter CMOS gates IC's may also be suitable candidates to work as linear amplifiers, despite the very scarce manufacturers application notes available on the subject.

The MM74HCU04M with its unbuffered single stage inverter gates, with a simplified internal CMOS transistor pairs arrangement, was chosen for this radio circuit to be used as a stable, low to mid gain analog amplifier in several RF and even on AF stages, due to its high noise immunity, high speed, low input capacitance, high input impedance, high fan-out capacity, and transfer characteristics.

This approach requires a clever design to handle potential instability and unwanted oscillation, maintaining each gate within a very narrow operating point in the so called "linear" region corresponding to the transition zone between the ON and OFF states.

Still, I am surprised to see these gates running at 30MHz (AM RF) and even at 55MHz (1st AM IF) and up to 85.84MHz (1st Local Oscillator). Surely these are selected devices to work in a stable way at that frequencies.

I am not sure about this design choice, considering the alleged (*) inferior sensitivity result when compared with well established designs using JFET transistors as RF amplifiers, for instance.

Still, this is an interesting radio model, unconventional in design, and apparently produced in limited quantities which would make it a collectible model.

 

(*) Based on some public videos comparing this model against a Sony ICF competitor of the era. Also, my initial tests are not encouraging, with poor MW sensitivity, average FM, and mediane SW, but it may be caused by my unit being defective, as I did not had the time to check it on a lab RF oscillator.

 

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The January 1986  Philips  Designer's Guide, High-speed CMOS has application circuits for unbuffered parts as oscillators. I actually have the copy I used in 1986. Page 115 offers an overtone  crystal oscillator usuing the 74HCU04 and suggests 3rd overtone 10MHz to 75 MHz and 5th overtone 50 MHz to 75 MHz crystals. Using them as cheap analogue amplifiers was well known in the late 1980s as the same had been done earlier using the 4069U unbuffered hex inverter, which is a simiar internal circuit. See page 11 of TI CD4068UB for the crystal oscillator, linear amplifier and RC (multivibrator) similar to the 74HCU04. It's 3V to 18V operation but considerably slower than the 74HCU04, about 5x slower at 5V and about 3 to 5x the input capacitance.

The Buffered parts have an additional output stage and can only be used for some kinds of oscillators. The earller 4000 series also had unbuffered parts that could be used with DC feedback to bias and while slower, they could work over 3V to 15V, or some makes or models up to 18V.

It was a well established technique years before 1988 in telecom and industrial applications. So rare in a domestic radio, but not unorthodox. Very cheap and low power, 2V to 6V operation and one package of 6 amplifiers is about the same cost and less space than six FETs.

The current Nexperia data for the 74HCU04 shows each gate is one complementry push-pull pair and page 6 shows the analogue test circuit.

The first circuit in 12. Application information (page 7) Fig 13 shows an inverting AC linear amplifier. I've used this circuit maybe more than 35 years ago. The other two applications are oscillators. It's never used as a logic inverter, the buffered part (without the U) is used for that.

The speed (transistion time) and thus frequency response is better as the supply voltage is raised, 110 ns @ 2V and 19 ns @ 6V.
Power dissapation is proportional to frequency x Supply Volts squared x 10 pf
 

Input capacitance is on 3.5 pF. So the main limitation would be noise. That's only an issue progressively for VHF, UHF and SHF. The noise in this device would be lower than regular LW, MW & SW bands.

 

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Certainly the circuit design needs to be careful for an 85 MHz local oscillator, especially with the nominal 3V supply. The end point would normally be 2V to 2.2V with two cells. So the only thing that surprises me is that it's not three or four AAA cells as the supply (3V or 4V end point). I expect The VHF would fail first and before the batteries are fully exhusted. That would be an easy test with an external PSU.

I'd be very surprised if the ICs were specially selected. It would have been a lot cheaper to produce than a Sony ICF2001D.

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Thanks for your comments and contribution, Michael. 

Ok, it is not that unorthodox then.

I had my fair share of CMOS digital projects back in the mid 70's, including designing analog and all kind of oscillator types around these unbuffered gates. But when most of the digital series were unbuffered, we used them primarly for their intended function in digital circuits. Buffered gates came a few years later, as far as I remember.

Please note that all the 74HCU04 datasheets clearly state the DC parameters (Vih, Vil, Voh, Vol) as well as propagation delays, common to digital gates, as expected for the 74xx series, be it buffered or unbuffered. There is no explicit or implicit indication that this HCU series are dedicated for analog or linear circuits. Nothing impedes the use of unbuffered gates in pure digital circuits, we have done it since long time ago on the initial 4000 series. Granted, these days we would avoid the use of such gates in digital circuits as we have better components for the job.

Looking to this subject from another point, taking these CMOS gates meant for digital operation (ON or OFF states with very low current consumption at those states), and then forcing them into class A operation may result in excessive current consumption when we think of a small pocket radio, even if the gates are run at just 1.9V or about (pls see the Philips schematic).

According to the schematics of this radio, the AM consumes around 50mA and FM around 40mA. These values looks a bit excessive for such a small pocket radio running on AAA dry cells. A good AAA alkaline cell with 1000mAh would be totally depleted after just around 13 hours of continuos operation.

Coming back to my radio issue with low sensitivity. I am not at home and far away from my lab, to be able to do proper testing. But now I found marks of battery electrolyte corrosion at the power jack and inside the battery compartment. I did not see this when I bought the radio because it was an online sell. Additionally, if I turn the voliume up, the radio powers off, as if the batteries were depleted (and it is not the case).

 

A few technical observations on this model:

In this Radio model, the MM74HCU04M Hex Inverter IC's are running at distinct and decoupled power supply lines, with voltages ranging from 1.7V to the 3V of the Battery line:

• Actively regulated lines are used for IC's 7105 2.1V, 7121 1.9V, 7122 1.9V, and 7212 1.8V. 
• RC passive network lines are used for IC's 7257 1.7V, 7500 1.7V.
• Variable Vcc voltage is used for IC 7501 from 0.5V to 1.5V.
• The 3VDC Battery line is used for IC 7284.

 

Partial parts listing and function:

• 7129    CXA1238M    AM/FM Stereo tuner (FM RF/MIX/OSC/IF/decoder, AM IF/decoder)
• 7132    HEF4007UBT    Dual complementary pair and inverter (Loop Ampl for Varicaps Tuning Voltage, 10.2V DC-DC Converter with 7130 TR BFS20)
• 7105    MM74HCU04M    Hex Inverter (AM LW-MW RF/AGC, AM Local/DX, 1ST IF 55.845MHz)
• 7121    MM74HCU04M    Hex Inverter (AM 1ST IF 55.845MHz, AGC Ampl, 2ND IF 450KHz)
• 7122    MM74HCU04M    Hex Inverter (AM 1ST Local Osc 55.99-85.84MHz, AM 2ND Local Osc 55.395MHz, AM MW/LW Osc 0.597-2.56MHz, SW 2.565-30.45MHz Osc for Pre-Scaler)
• 7212    MM74HCU04M    Hex Inverter (AF L-R Pre-Amp, FM IF 8:1 Divider 1.3375MHz)
• 7257    MM74HCU04M    Hex Inverter (AF L-R 5KHz Suppression filter, AF Mute, News-Music Filters) 
• 7500    MM74HCU04M    Hex Inverter (AF L-R Ampl, News-Music Selector, Bridge R-Inverter when internaal Speaker in use, Electronic Volume Control)
• 7501    MM74HCU04M    Hex Inverter (Dedicated to AF L-R Volume Control by changing the Vcc pin 14 from Min=0.5V to Max=1.5V)  
• 7284    MM74HCU04M    Hex Inverter (Power Control ON/OFF with 7283 TR BCX69-25, Low Voltage Switch, Alarm/Sleep)
• 7133    TD7101F        ELC Prescaler For Digital Synthesized Tuner (FM/SW Pre-scaler)
• 7281    LA4535M        Stereo Power Ampl for Headphones 
• 7410    SMC62A32    Epson 4-bit MCU (Clock/Date/Alarm with Clock Display LCD, Electronic Volume Control)
• 7401    TC9308AF-38    Toshiba 4-bit MCU (DTS/Radio Management, PLL, Radio Display LCD) 
• 7101    BFS20        FM/AM SW Antenna Splitter
• 7107    BFS20        NPN TR (1ST Mixer)
• 7108    BFS20        NPN TR (2ND Mixer)

 

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