# A Wideband High-Voltage Power Amplifier Post-Linearizer for Medical Ultrasound Transducers
## 3 Results and Discussion  
### Experimental Performance Verification of the HVPA with Post-Linearizer  
The impedance variation of the HVPA was found to be offset by the post-linearizer circuit, as described in the previous chapter. In order to verify this claim, the gain and gain deviation as a function of HVPA input power, and the frequency with and without the post-linearizer were measured, respectivly. Figure 5 describes the measurement method. Input signals from an arbitrary function generator (AFG3252C, Tektronix Inc., Beaverton, OR, USA) were applied to the HVPA with or without the post-linearizer and DC power supply (E3631A, Agilent Technologies, Santa Clara, CA, USA). Subsequent to passing through the 20 dB power attenuator (BW-S20W20+, Mini-circuits, Brooklyn, NY, USA), the output waveform was measured in an oscilloscope (MSOX4154A, Keysight Technology, Santa Clara, CA, USA). A power attenuator was used to reduce the high-voltage when measuring the results, as the maximum voltage was restricted to 5 V in the 50 \(\Omega\) input impedance setting of the oscilloscope. Figure 6a-d illustrates that the measured gain and gain deviation of the HVPA with and without the post-linearizer, respectively. 10 MHz, 5-cycle input power signals were applied to the HVPA when 1, 2, 4, and 5 V DC voltages were applied to the post-linearizer. The output signals were then measured in order to calculate the gain and gain deviation of the HVPA with and without the post-linearizer. The input 1 dB compression point (IP\({}_{\mathrm{1 dB}}\)) defines the limited linearity index of the power amplifier, which is the point at which the amplifier gain is 1 dB below the initial gain [30; 32]. Thus, the IP\({}_{\mathrm{1 dB}}\) of the HVPA with and without post-linearizer were measured and compared in order to evaluate the linearity of the HVPA without and with post-linearizer, respectively. Figure 5: Measurement setup to calculate the gain as a function of HVPA input power and frequency with and without the post-linearizer. As shown in Figure 6a, the gain of the HVPA with post-linearizer when 1 V and 2 V DC voltage were applied to the post-linearizer increased to \(-\)4 dB\({}_{\rm m}\), and then decreased to \(-\)1 dB\({}_{\rm m}\) input power. However, the IP\({}_{\rm 1 dB}\) of the HVPA (14.21 dB\({}_{\rm m}\)) was lower than that for the HVPA with post-linearizer when 1 and 2 V DC voltage were applied (16.11 dB\({}_{\rm m}\) and 16.81 dB\({}_{\rm m}\). respectively). As shown in Figure 6b, the gain deviation of the HVPA, and that of the HVPA with post-linearizer when 1 V and 2 V DC voltages were applied, yielded instability up to approximately \(-\)1 dB\({}_{\rm m}\) input power. The gain deviation of the HVPA (\(-\)0.40 dB at \(-\)4 dB\({}_{\rm m}\) input power) was lower than that of the HVPA when 1 V and 2 V DC voltages were applied to the post-linearizer (\(-\)0.53 dB and 1.34 dB at \(-\)4 dB\({}_{\rm m}\) input power, respectively). However, the gain deviation of the HVPA with the post-linearizer when 1 V and 2 V DC voltages were applied (\(-\)0.36 dB and 0. 28 dB at 17 dB\({}_{\rm m}\)) was lower than that of the HVPA without post-linearizer (\(-\)1.23 dB at 17 dB\({}_{\rm m}\)). The gain of the HVPA with post-linearizer yielded moderate stability between \(-\)1 dB\({}_{\rm m}\) and 26 dB\({}_{\rm m}\) when 1 V and 2 V DC voltages were applied to the post-linearizer. These results confirm that, for 1 V and 2 V DC voltage, a minimum input power of \(-\)4 dB\({}_{\rm m}\) is required to obtain efficient operation of the post-linearizer. Figure 6c,d shows the measured gain and gain deviation of the HVPA with and without linearizer, respectively. As shown in Figure 6c, the overall trend of gain of the HVPA with post-linearizer for the cases of 4 V and 5 V DC applied voltages is flatter than that of the HVPA without post-linearity.