Fig.1 A simulation result of "Low powered AM modulation" circuit on LTspice (4).

(LTspice(4) is a spice simulator developed by Linear Technology inc.)

Fig1. is an AM/FM modulator component that can generate AM modulated signal voltage or FM modulated signal voltage. It's embedded in LTspice (4) .

In this simulation, I used it as an AM modulator.

The directive "SINE(40m 40m 1K)" means the amplitude signal voltage as input audio signal is 40mV and DC offset voltage 40mV. Input signal of sine wave 1KHz is swinged on the DC offset voltage 40mV, and is modulated with 1MHz carrier frequency.

In this case, the AM modulation rate = 100% and quality of the modulated output signal voltage is very good and it is almost perfect.
On the FFT of Fig1, there is almost no distortion in this work and is almost ideal work.

Fig.2 Block diagram of "Low powered AM modulator"

Fig.2 is a block diagram of "Low powered AM modulator" that explains how it works.

I thought it out by myself from the equation (1) of AM modulation signal voltage.

Vout=(Vdc+x(t))*(Vc*sin(ωc*t)) ... (1)

Here,

x(t): baseband (audio) signal voltage to be fed to the AM modulator .

On the Fig.1, x(t) = Vin = V3 = V0*cos(2*π*f0*t) ...(2)

Vdc : DC offset voltage V2. Vdc is often defined as DC 1V in other explanations.

Adder: Analog signal adder input Vin and V2 and output the added voltage signal as V4.

V1 : Oscillator of sine wave to generate carrier signal for AM signal.

V1 = Vc*sin(ωc*t), ωc= 2*π*f1*t ...(3)

Mixer (Multiplier) : Analog multiplier. It inputs V4 and V1, then does multiply V4*V1,

and output V4*V1 to Vout

Vout : Output signal voltage

As described above, the principle of generating AM signal is very simple.

Once we know the principle beforehand, it makes designing or building AM transmitter/transceiver be quite easier.

Fig 3. Collector AM modulation method at the Final transistor used in RJX-601

(Copyright by "Matsushita Denki Corp." in about 197x(?)-198x(?))

RJX-601 is very popular products, many people of Amateur radio licensed love it.

It can send 1W or 3W of AM or FM E/M wave on the frequency 50MHz to 54MHz.

The specification was the best one on those days.

However it has some subjects on electronic work in sending AM modulated signal.

When RF power selection is set to 1W, AM modulation is good.

But it is set to 3W, it generates low modulated AM.

This phenomena was so called "Minus modulation" .

Why "Minus modulation" occurs ?

Following is my analysis of the "collector modulation method at the final transistor".

Fig.4 Abstract circuit of BJT transistor's collector AM modulator

Fig4. is an abstract / equivalent circuit of BJT transistor's collector AM modulator.

Please watch Fig.4 carefully comparing with Fig.1.

(1) Optimized DC bias voltage of VB is fed to base terminal of the final transistor.

This bias voltage must be set to class AB or class A to amplify with good linearity.

Class C is impossible to use in this application. [2]

(2)RF sine wave voltage source such as 50.6MHz is fed to base terminal of the

TR(Transistor). Assume Z of Vrf here = 0 ohm and base terminal has some resistor

value such as 500 ohm.

(3)Audio Power Amplifier is required to amplify small signal from a microphone to become enough power such as 1W to 5W and drives the 1st side of "Audio power modulation trans".

(4) Vcc = 12V to 13.8V is fed to the 2nd side of "Audio power modulation trans" and amplified audio signal AC voltage rides on the DC Vcc line. This audio modulation signal is connected to "Zc".

(5) "Zc" is assumed to use ferrite beez such as FB-225. The ferrite beez acts as resistance of hi-Z and inductance value is very low on RF frequency domain.

It makes the TR to be stable as amplifying device.

Traditional circuit had used LC tuned tank circuit in the application of vacuum tubes. But LC tuned tank circuit makes the TR works to be very unstable.

Because the TR cannot work on such hi-Z stable, it causes abnormal RF oscillation and TR goes to heaven. It often causes permanent break downs of very expensive TR.

(6) LC Z-matching section must be adjusted to be terminal impedance properly such as 50 ohm. We can never use LC Hi-Z tank circuit here.

(Be careful. LC tank circuit is for vacuum tubes but not for TRs.)

(7) Feeding audio amplified signal rides on Vcc=DC 12V to 2 stages of TRs audio amplified is not correct theoretically according to the principle of AM modulation method mentioned above (See the equation(1)).

Fig.5 Visualized work of AM modulation on BJT TR

Fig5. is visualized picture of AM modulation on BJT TR on the final stage of RF power amplifier.

This explanation is based on the traditional design theory of TR amplifier with "Load Line method" that was known in 1964.[1]

In this load line design method, TR bias point is set to saturation area of Collector Current "Ic".

Now assume Vcc=12V and there is no audio amplified signal on Vcc on the Load Line.

50.6MHz RF sine voltage signal comes in to TR and amplified along with the Load Line.

There is still no problem.

In this status, input audio signal. Then the Voltage of collector terminal of TR is swinged deeply by the amplified audio signal voltage.

Since Vcc is swinged by the amplified AC voltage, and Load Line is moved by it from left of Vcc and/or to right of Vcc widely.

This means proper bias point of TR moves dynamically and the proper bias to become bad bias point.

As the characteristics of BJT TR, Ic increases with very small amount and it is not linear work with big distortions. The TR can't work as an analog multiplier on this Ic saturated area.

This is the reason why "Minus modulation" occurs.

There is enough reason traditional designed circuit caused "Minus modulation" when 3W output is specified by the power selectable switch.

There was a reasonable design error. :-)

How should we correct the traditional design to become well-designed one?

From the equation (1)', Fig.6 can be introduced as followings.

Fig. 6 AM modulation by Analog Multiplier

Fig. 6A AM Collector modulation by BJT TR 2N2222 used as an "Analog Multiplier"

(This types of design works with bad quality of AM with distorted AF tone.

The characteristics of BJT TR as an "Analog Multiplier" is not always good.)

Vout=(U0+x(t))*(Vc*sin(ωc*t)) ... (1)'

Here,

x(t): baseband (audio) signal voltage to be fed to the AM modulator .

On the Fig.6, x(t) = V3 = Va*cos(2*π*f0*t) ...(2)'
V3 is actually audio power amplifier to supply modulation Voltage source with DC offset U0 by the Vcc line (eg. 12V).

U0 : DC offset voltage V2. (=Vcc=12V)

Adder: Audio power trans here works as an adder.
It inputs V3 and U0, add them and output the voltage signal Vin to Multiplier.

V1 : Oscillator of sine wave to generate carrier signal for AM signal.

V1 = Vc*sin(ωc*t), ωc= 2*π*f1*t ...(3)'

Mixer (Multiplier) : Analog multiplier. It inputs V1 and Vin, then does multiply V1*Vin,

and output V1*Vin to Vout

Vout : Output signal voltage

The equation (1) and (1)' have the same meaning as AM modulators. The deference between them is that they are used in the low power earlier stage of the transmitter or used in the high power last stage.

I've heard that the principle of the AM modulation method by Fig. 6 had been used in the "Plate modulation method" by Final vacuum tube.

I"d like to think, maybe, understandings for multiplier's characteristics of vacuum tube had not always been understood well in those days.

From as my beginners view, TR collector AM modulation method is simply seemed to be exchanged the vacuum tube of plate modulation to a BJT transistor instead.

However, there are big difference of electric characteristics between the vacuum tube and BJT TR.
BJT TR is known as the active device that has the characteristics of CCCS(Current Controlled Current Sources).
Beside, the Vacuum tube is known as the active device that has the characteristics of VCVS(Voltage Controlled Voltage Sources.)

Unfortunately, I've not heard or read of the characteristics as a Multiplier of the old fashioned vacuum tube.
I'm wondering if nobody has been interested in the equation (1) or (1)' in past so long years ?

Once we notice the characteristics of a multiplier is essential for AM modulation from the equation (1) or (1)', we can fix the problem as a subject of Math.

As next step, I thought is about how can I use a BJT transistor as a multiplier.

Generally, BJT transistor's amplifier is designed on the "Load Line" where the collector current is saturated. [1]
In this saturated collector's current working area, BJT TR cannot work as an analog multiplier because the Ic is saturated and increasing VCE cannot increase collector current.

By the way, how does BJT TR work in front of Ic saturated area ?

Fig.7 BJT TR's characteristics before collector current saturated.

Fig.7 shows the BJT TR's characteristics that Ic increases according to VCE increases before Ic is saturated.

Using this Vce area of BJT TR is not recommended by the old fashioned design book.

In the book, Ic was saturated in very low Vce such as 0.3 - 0.5 V.

How it work in the actual BJT TR such as 2SC1815 very popular transistor in Japan ?

Fig. 8 Ic vs VCE characteristics of 2SC1815

From the DC sweep of simulation result of Fig.8, BJT TR 2SC1815 has wide range Vce = 0 to 5V with straight linearity of Ic increase.

This implies one (1) 2SC1815 can be used as an analog multiplier.

Fig.9 1 TR 2SC1815 (as an analog multiplier) AM modulation works

Fortunately, it works well. This circuit has benefit that it is not necessary to use a big audio modulation power trans. LM386 can drive the final TR 1815 directly, and AM modulation works well.

Next topics is also serous subject on so called "Base AM modulation method".

Fig.10 Base AM modulation transmitter not work at all

"Base modulated AM method" is very famous even now and the circuit of Fig.10 was found on the internet that seemed to be designed by a professional engineer.
Unfortunately, I could not get any good result on the spice simulation for it.

I had some experiences that the base modulation by a BJT TR didn't work well on the actual circuit. It could never get good result also on it.
I knew that these unwanted situations could be re-confirmed on the LTspice.

Fig.11 After improvement of the circuit of Fig.10

I tried to fix the design errors that I found on the Fig.10 base modulated AM transmitter.
And I managed to work it as Fig.11.
But the quality of modulated RF signal was so dirty.

I know Base AM modulation method has been very famous design but I have no solution to get good quality of AM modulated RF wave even now.
I have lost interest on this method until now.

New design by CQ Pub.inc used Gilbert-cell configured BJT TR AM modulation.

Fig.12 First ever Gilbert-cell type AM modulator
(Copyright by "Transistor Gijyutu" by CQ Pub.inc. 2015)

Analog multiplier configured by Gilbert-cell has invented in US, 1968 as far as I know.
It has very good quality as an analog multiplier and implemented in many DBM (Double Balanced mixer) ICs.
Fig.12 circuit is the best result of AM modulation method configured by BJT TRs I've ever seen.

Fig.13 Works of C class amplifier

(Copyright by USAF 1963, Mr. Jeff Quitney, Mr. Youtube)

I cannot find application examples of C class amplifier recently.

I know C class amplifier works with big distortion then it is used to generate harmonic waves as distorted RF output signal such as x3, x4, x5 frequency multiplied.

I think it is impossible to generate good quality of AM modulation signal from the principle of AM modulation, but very famous Radio magazines for radio fans had been told us that C class amplifier worked very well for AM modulation even until now.

I have no experience that C class amplifier without bios voltage got any good quality of AM. I have no idea to realize to use C class amplifier for AM modulation.

From the view of the principle and the characteristics of C class BJT TR amplifier, it's impossible.

But they say "C class amplifier is very easy to make AM transmitters."

I cannot find any valid evidence for them.

There are lots of circuit examples of C class ( no bias) amplifiers on the internet even until now but I cannot find any reasonable result from my actual experiments and my calculation by Math. And every simulation by LTspice result tells me that it doesn't work at all.

Fig.14 Works of AB class amplifier

(Copyright by USAF 1963, Mr. Jeff Quitney, Mr. Youtube)

Class AB is set in RJX-601 on Fig. 2. Maybe, they knew that Bias voltage was essential for AM modulation by their experiences.

Fig.15 Works of A class amplifier

(Copyright by USAF 1963, Mr. Jeff Quitney, Mr. Youtube)

Class A amplifier requires lots of power consumption even when it is idling. So it is not good for AM transmitter.

This video tells me vaccum tube was used in the linear amplifier, and vacuum tube has a characteristics where there are linear VCVS(Voltage Controlled Voltage Source) working area.

It also implies vacuum tube has a good characteristics as an analog multiplier.

This characteristics gives reasonable reasons why "plate AM modulation method" works very well. Beside, BJT TR doesn't have a good characteristics as an analog multiplier. This is big difference between BJT TR and vacuum tube.

Vacuum tubes are often low-estimated as old-fashioned devices but they still have very good linearity and strongness for heavy duty usage for high power RF amplifiers.

Reliability of vacuum tube can be much better than TR's in the high power applications.

Fig. 16 is product named "PanaSky 6". I have a few experience to use it when I was a school student. It could send 10W high power AM modulation by vacuum tubes.

An elder student was saying "Don't low-estimate it. It has very good performance."