Miscellaneous Circuits for Idea Purposes
Introduction
Here are some varied circuits which were part of my spring and summer 2005 experimentation. They are meant to be idea circuits for spawning your own work. Many need work to improve performance.
For my radio projects, I prefer an AC output wall wart transformer so that better DC filtering can be used. This is a bare-bones 12 volt, low current DC supply for a small receiver.
A LM386 audio amplifier with a active tone control providing ~ 10 dB boost/cut to the circuit. The pots must be wired to Q1 as shown in the inset photo so that the desired tone control increases as the pots are turned from left to right. This circuit is not the quietest I have built, but is suitable for a popcorn "basic" AM receiver.
The power supply and audio stages. The LM386 audio amplifier and AF preamp have a separate, small, copper ground plane which is connected by a single heavy gauge wire to the main receiver ground buss. This is to help prevent ground loops and hum.
A homebrew differential mixer. The IF transformer needs some work to reduce losses. Suggestions include more turns of wire and possibly trying a 61 or 63 material ferrite core. A reference to for this circuit is EMRFD. I found a couple websites with homebuilt Gilbert Cell mixers and also suggest these for reference.
In the photo with the experimental circuit on top of the hand drawn prototype schematic. A double tuned 10 MHz band pass filter can be seen to the right of the copper RF shield.
Another view of the differential mixer and local oscillator.
A simple, crystal controlled 10 MHz local oscillator and buffer.
This is a 10 MHz, tuned input VPA with variable gain via a 10K linear taper potentiometer. This simple gain control circuit could be applied to other cascode JFET VPAs.The output impedance is 1K to match the input impedance of the experimental differential mixer shown earlier. Note the 3 foot (91.44 cm) whip. The L2 transformer could be changed as shown in the Figure 8 schematic on the 2005 More Active Antenna Experiments page. I recommend a 4 foot or greater length whip for VPA projects.
Active antenna prototype.
A double tuned filer designed to have an input and output impedance of 1000 ohms.
A test receiver I use for testing mixers and filters. It has the audio stages and power supply shown above. Note the RCA connector below the speaker holes. This is an audio line-out jack which allows the receiver to be connected to my PC sound card. Digital filtering software can then be utilized for experiments. Check out http://www.weaksignals.com/
Another interesting gem that W7ZOI sent me last winter to experiment with. It was designed with the software from EMRFD. Do not be tempted to use low Q, pre-fabricated inductors such as epoxy-resin coated axial units or you will have serious attenuation of the signal. I used a T50-2 core. My insertion loss was 3.2 dB, the 3 dB bandwidth was ~300 KHz. It tuned very sharply, however, had a nice, clean pass band when swept. I temporarily removed the variable capacitors and measured them to get some idea of what capacitance was needed to resonate 10.0 MHz. The measurements are shown; around 50 pF (which is what the software calculated). Nice filter.
I built and presented a Norton RF preamp circuit back in 1999 on this page using a ferrite torroid as the inductor. Fellow British Columbian experimenter, Markus Hansen, VE7CA, built this version in Spring 2005. These Norton variations work great and may have a flat response well up into the 6 meter band. I have also built these quiet RF amps using various low-cost transistors including the 2N3904 with reduced current.
The VE7CA ugly bread boarded version. Thanks Markus! Jim Kortge, K8IQY, uses the Norton noiseless feedback RF amp in his excellent 2N2/20 receiver. Jim uses an LED to help bias the base and thus his amplifier can be used with a supply voltage as low as 6 volts DC. Jim is an incredibly talented designer and builder. Please study his schematic and consider experimenting with this fascinating amplifier. It has low noise and works with just a modest performance loss at lower current levels, making it a consideration for popcorn projects.
Mark Kirkwood,N5AYC, Audio Power Amplifier
Mark designed a small power audio amplifier for QRP receivers.It can put out about 4 Watts into a 4 ohm speaker and is stable. The input contains a simple diplexer to follow a diode ring detector. Mark used a basic approach to keep his design functioning well. Many thanks to Mark, N5AYC for sending this design along!
Note: R15 is a 10K potentiometer.
Astable Discrete Transistor Multivibrators
I took this sharp photograph of the output waveform of a typical 2 transistor astable multivibrator. Note the gradual rise in voltage which occurs as a result of the capacitor connected to the output transistor collector charging up. I found connecting a standard 2 transistor multivibrator to the 4017 often resulted in improper sequencing of the LED chain. Adding a resistor and a diode to the Q2 collector cures this problem and triggered the 4017 as desired in my experiments. This modification is only required on the transistor from which you take your output, but both Q1 and Q2 can be modified as shown to obtain a more "square" output waveform for both transistors when 2 outputs are required.
The output waveform should rise quickly to best trigger the 4017. Make R1 10-22K or greater in value than R2. Thus, as the capacitor charges through R1, the diode is reverse biased and effectively disconnects the capacitor connected to the Q2 collector. This gives a quicker rise time to the output. You can vary R1 slightly to fine tune the frequency of your oscillator. The capacitor connected to Q2 collector was 3 -5 times smaller than the capacitor connected to Q1 collector in my versions. The load resistance (RC) for Q2 is approximately the sum of R1 and R2 in parallel. A practical example is the 3 volt astable multivibrator schematic shown earlier.
Astable Multivibrators at Radio Frequency
I have read that multivibrators can oscillate into the radio frequency spectrum. Resistor-Capacitor (RC) and crystal oscillator versions were built and briefly examined.
This RC oscillator had an output at 7.030 MHz. The capacitors were NP0 ceramic types with values obtained by placing a 100 pF plus a 56 pF in parallel. As recommended by the late Courtney Hall who wrote several electronics books for the Howard Sams publications in the 1970s, the collector resistance was decreased to a very low value. This serves to help minimize the effects of stray circuit, component and load capacitance on the frequency stability. The circuit is run at 1.5 volts to reduce the current draw.
As shown, the circuit current draw was measured between 33-35 mA depending on the condition of the 1.5 volt battery used to power it. The circuit will not oscillate unless each base bias resistor is connected to the transistor collector terminal as shown. This type of multivibrator is referred to as a non-saturating astable multivibrator as it is not possible for the transistors to stay in the saturated condition.
The output waveform of the 7 MHz RC oscillator. It was far more frequency stable than I thought it would be, however, formal stability testing was not performed. The waveform is more sinusoidal than I expected. This may be partly due to the low pass filtering effect of my oscilloscope probe.
A 10 MHz crystal oscillator. Below is a photograph of the circuit breadboard with a 270 pF capacitor used instead of the 120 pF shown in the schematic. The 10 pF capacitor could also be a small trimmer capacitor to adjust the frequency slightly. The output waveform is below the photograph.
The output waveform of the 10 MHz crystal oscillator.
Conclusion
I hope that you have some fun experimenting with these and other circuits. Feedback is welcome and surprisingly rare considering this web site gets well over 1000 hits per week. Best bits! Todd, VE7BPO
