In an earlier blog (Comparing Op-Amp & Video Amp Topologies, Part 1), we examined the bandwidth and phase requirements for processing video. In the most recent blog, we looked at the AC and DC requirements of the circuitry. We left off with our discussion of video amplifiers by considering the value of a DC restoration circuit as used with black and white and early color TVs (Comparing Op-Amp & Video Amp Topologies, Part 2).
The simple amplifier with one voltage gain stage as illustrated in Figure 1 may be further modified.
This amplifier could be further improved by using a double emitter follower for Q3 so that there is more reverse bias voltage on Q2 such that its collector-base capacitance is reduced. Alternatively, a Darlington transistor may replace Q3, with the first transistor of the Darlington biased to a reasonable non-starving collector current.
As shown in Figure 1, with no signal at the input, the collector base voltage at Q2 is about 0.7 volt due to the turn on voltage of Q3. Using a double-emitter follower will raise the collector base voltage of Q2 to 1.4 volts, and most likely load resistor RL will have to be reduced slightly in value to maintain the same offset voltage. Another advantage in raising the reverse collector to base voltage on Q2 is that the negative output swing will increase, which may be useful for applications where the more than 1 volt peak into a 75Ω load is required at Vout.
It is advantageous to provide a DC level-shifting circuit via Q3 and other devices to increase negative output swing. For example, a zener diode may be in series with the emitter of Q3, with the cathode connected to the emitter of Q3 and the anode then connected to RF, RE5, and the 75Ω output resistor. Note that load resistor RL will have to be reduced in value appropriately.
Another improvement to this amplifier is to replace RE5 with a current source circuit such as an NPN transistor to provide better linearity in the emitter follower stage.
Eventually, a version of this amplifier was available in an integrated circuit via the Signetics NE5539. In this op amp, the output stage is a double transistor emitter follower, and the input stage included a pair of NPN emitter followers into the differential pair amplifier. The input NPN emitter followers allowed the differential pair transistor to have another 0.7 volt of collector base reverse bias. The output of the differential pair amplifier via a resistive load was connected to the input of the double transistor emitter follower circuit.
Figure 2 shows an alternative circuit for a video op amp, and the voltage swing is still limited to that of the circuit in Figure 1. This amplifier was used in the Sony BE-3000 video editor.
The amplifier above uses a fourth transistor Q4, which forms a second voltage gain stage. Because of the extra voltage gain stage, Miller compensation is used to allow for stability and flat frequency response. Tested under open loop conditions, the gain at 10 kHz to 50 kHz was measured at about 220 V/V, with a -3 dB frequency of 225 kHz. The gain bandwidth product is then about 50 MHz (220 X 225 kHz).
The Miller multiplier capacitor Cc with Q4 forms a pole calculated at about
Note if the collector base capacitance of Q4 is taken into account, which is roughly 2 pF, the calculated open loop pole will be about 176 kHz.
In comparison the circuit in Figure 1 has an open loop pole is set at approximately fc = 1/(2πRLCc), which is typically in the megahertz region.
With Q1 and Q2 = MPSH10, Q3 = MPS2222, and Q4 = 2N4126, the measured differential phase was ~0.25° and differential gain was ~0.1%. Frequency response was flat to 4.2 MHz easily within 0.2dB.
This op amp was used as an output amplifier for the Sony BE-3000 video editor's video switcher/effects board. And although the differential phase is about a quarter of a degree, this op amp was one of many in the video switcher board, and the main focus was that after the video signal had traversed through a multitude of other video amplifiers the net output distortion was still low enough for professional broadcast video purposes. That is, other amplifiers in the chain could have a complementary distortion shape which partially cancelled the differential phase or differential gain distortion.
In the next blog in this series, we'll look at another video amplifier similar to the one used in another piece of professional video equipment, the Ampex AVR-3 VTR. We'll see what the frequency response looks like by looking at some composite video waveforms.